Do parasitic signals affect the performance of pump laser diodes?

Parasitic signals are usually undesirable elements and most often unavoidable too. When parasitic elements are encountered in sensitive components like pump laser diodes, you need to take precautions to get rid of these parasitic signals. Otherwise, parasitic signals can lead to crosstalk, interference, and unreliable operation. That’s where pump laser protectors come into the picture.

Why do we need to protect the pump laser from parasitic signals?

Pump lasers and pump laser diodes are the components that require having high reliability. However, if we keep allowing parasitic signals to reflect into the laser, it can lead to errors, performance degradation, and unstable operation. Therefore, pump laser protectors play a great role in ensuring the safe and reliable operation of pump laser diodes.

Now, you must be wondering what exactly pump laser protectors are. Let’s find out.

What are pump laser protectors?

Pump laser protectors are passive components that are specifically designed to protect the laser’s center wavelength by preventing parasitic signals from being reflected back in the laser. Besides, they also allow maximum transmission from discrete pump laser diodes that are fiber-coupled. Since pump laser protectors filter out parasitic signals by blocking them, these passive components are also known as pump laser filters.

What are the applications of pump laser protectors?

Pump laser protectors find their use in a variety of applications, such as:

Fiber amplifiers: Can be used with fiber amplifiers that are used to boost optical signals directly without converting them into electrical signals.
Fiber laser: Can be utilized with solid-state type laser that utilizes optical fiber as the gain medium and is widely used for material processing, telecommunication, etc.
Testing: It is not easy to distinguish the effects of parasitic signals, which made it many times difficult to measure the performance of certain fiber optic components. Pump laser protectors can help with testing.
Instrumentation: Can help achieve a high signal to noise ratio

What Properties Do Pump Laser Protectors Have?

When buying pump laser protectors, you should verify if they:

Have low insertion loss
Can handle high power
Offer high isolation
Are highly reliable
Have excellent temperature stability
Are highly affordable

Where Can I Buy High-Quality Pump Laser Protectors (Filters)?

If you are looking to buy high-quality pump laser protectors, look no further than DK Photonics. We can provide you with pump laser protectors in different specifications. Even if you can’t find pump laser protectors with the specifications you are looking for, you can place a custom order and we will facilitate you with it. All you need is to contact us and share your custom specifications.

Pump and signal combiner for bi-directional pumping of all-fiber lasers and amplifiers(10)

7. Signal feedthrough of the fiber combiner

Besides the pump power handling and the pump coupling efficiency of a fiber combiner, it is important for fiber laser and amplifier applications to maintain the optical properties of the signal light propagating through the fiber combiner. In particular, during the fabrication of the fiber component, externally induced mechanical stress and perhaps a marginal fraction of thermal diffusion of the core dopants [19] can result in a high signal insertion loss in conjunction with a degradation of the signal beam quality. This behavior was expected for large mode area DC fibers with a very low core refractive index (NA ~0.06), and therefore possible beam quality degradations of the signal feedthrough light was investigated (in Section 7.1).

The uninterrupted signal core in the fiber combiner provides the possibility of passing a signal beam through the combiner in forward and backward direction. However, in the case of a backward propagating signal, the pump diodes need sufficient protection against the signal. Thus, in Section 7.2 we investigate the signal to pump isolation of a 4 + 1×1 fiber combiner in a fiber amplifier setup.

7.1 Signal insertion loss and beam quality

In order to determine possible beam quality degradation and a signal insertion loss caused by the signal feedthrough of the combiner, the setup depicted in Fig. 14

Fig. 14 Setup for beam quality measurements, TF: target fiber, PBS: polarization beam splitter.was used. A signal at a wavelength of 1064 nm was launched into the core of a 2.75 m long Ytterbium-doped DC fiber (Nufern YDF-25/250), which is specified with a signal core diameter of 25 µm (NA 0.06) and a pump core diameter of 250 µm (NA 0.46). Thus, the parameters of the passive TF of the combiner were matched to the active fiber. The coiling diameter of the active fiber was 12 cm to maintain near diffraction limited beam quality [20]. The transmitted signal had a power of about 200 mW and was propagating in reverse direction through the fiber combiner. The beam quality measurements were carried out with a Fabry-Perot ring-cavity. With this cavity it was possible to determine the power fraction in higher-order transversal cavity modes with respect to the Gaussian TEM₀₀ mode by scanning the length of the ring-cavity over a free spectral range (FSR). A detailed description of the measuring setup can be found in Ref [21]. Due to the use of a polarization sensitive beam quality measurement, a half- and a quarter-wave retardation plate in conjunction with a polarization beam splitter (PBS) were used. The determined polarization extinction ratio was better than 17 dB after the propagation of the signal through the active fiber and the fiber combiner.

Before the fusion splice between the active fiber and the 4 + 1×1 combiner, the power in higher-order modes of the active fiber was determined. This measurement served as a reference beam quality for the active fiber. The mode scan in Fig. 15(a)

Fig. 15 Normalized transmitted intensity through a premode cleaner as a function of the ring-cavity length in units of a free spectral range for (a) the reference beam and (b) the signal feedthrough beam of a 4 + 1×1 fiber combiner.

shows the logarithmic normalized intensity over a free spectral range for the reference beam with a power in higher-order modes of 3.1%. This results in a fundamental fiber mode power of at least 96.9% for the reference beam. For the signal feedthrough of the fiber combiner, a power in higher-order modes of only 5.1% was found (Fig. 15(b)).

Consequently, the signal feedthrough fiber (0.7 m long TF) only led to an increase in power in higher-order transversal modes of maximal 2%. Furthermore, it must be considered that additional power transfer to higher-order transversal modes can also be caused by the fusion splice between the active DC fiber and the TF. Hence, good preservation of the signal beam quality, in conjunction with the low signal insertion loss of less than 3%, provides an excellent high power fiber component for monolithic fiber laser and amplifier systems.

2015-Fiber Optic Communication Collimators Market Forecast

Fiber optic collimator lens arrays are forecast with strong value-based growth rates of more than 30% per year (2014-2019)…

Aptos, CA (USA) – March 23, 2015 — ElectroniCast Consultants, a leading market research & technology forecast consultancy addressing the fiber optics communications industry, today announced the release of a new market forecast of the global market consumption and technology trends of small beam collimating lens assemblies in fiber optic communication (including telecommunication, datacom and cable TV) passive and active/integrated (hybrid) components/devices.

The market study covers single lens assemblies, 2-12 lens arrays, and arrays with more than 12 lenses. Both of the lens array categories are forecast with strong growth rates of more than 30% per year (2014-2019). Single lens fiber optic collimator assemblies held the global market share lead, with over 80% in 2014.

“Collimator lenses are used in a variety of photonic products; however this market study forecasts the use of micro-sized collimator lens assemblies, which are used specifically in optical communication components/devices(such as 8CH LGX CWDM Module). Fiber optic collimator lens assemblies serve as a key indicator of the growth of the fiber optic communication component industry,” said Stephen Montgomery, Director of the Fiber Optic Component group at the California-based consultancy.

ElectroniCast defines lens assemblies as “loose” lenses (one or more), which are attached to an optical fiber or fitted/attached into (or on) a planar waveguide/array substrates or other device(s), such as a ferrule, for the purpose of collimating light for optical fiber communication.

The global consumption of fiber optic collimator lens assemblies, which are used in commercial optical communication applications, reached $287.2 million in 2014, an increase of 8.7% over the previous year.

Consumption is based on the geographical (region) location where the lens assembly is first used into (the) higher-level component or module package; therefore, ElectroniCast forecasts that the Asia Pacific Region will hold the market share lead for most of the timeframe covered in the forecast period. America, led by the United States, is forecast to remain in the 2^nd-place market position until 2019. Europe is forecast to maintain moderate-to-strong growth, as the region is steadily involved in value-added building (and use) of sub-assemblies and equipment. Market forecast data in the ElectroniCast report refers to consumption (use) for a particular calendar year; therefore, this data is not cumulative data.

DK Photonics – www.dkphotonics.com specializes in designing and manufacturing of high quality optical passive components mainly for telecommunication, fiber sensor and fiber laser applications,such as 1064nm High Power Isolator,1064nm Components, PM Components, (2+1)x1 Pump Combiner,Pump Laser Protector,Mini-size CWDM,100GHz DWDM,Optical Circulator,PM Circulator,PM Isolator,Fused Coupler,Mini Size Fused WDM.

The Asia Pacific region is the leader in value of the fiber optic communication collimators market; however, the American region is forecast to take the lead in 2019 …

Fiber Media Converters in Private Datacom Market Forecast (March 2014)

Fiber Media Converters in Private DatacomMarket Forecast (March 2014)

According to ElectroniCast, the global use of fiber media converters in private datacom networks is expected to reach $1.29 billion in 2014…

Aptos, CA (USA) – March 20, 2014 —ElectroniCast Consultants, a leader in fiber optic market research, announced the release of a new market analysis of the worldwide use of fiber optic / Fiber media converters in private data communications. A fiber media converter is a networking device that makes it possible to connect two dissimilar media types such as copper with fiber optic cabling, as well as (different) fiber-to-fiber (F2F), such as multimode to single mode optical fiber.

The worldwide value for selected fiber media converters used in private datacom networks reached $1.07 billion in 2013. The consumption value is forecast increase with strongly rising quantity growth partially offset by declining average prices.

The EMEA and the APAC regions are forecast for double-digit consumption value growth during the timeline covered in this study (2013-2018); however, the American region’s growth is forecast to “flatten” and eventually turn to negative. The worldwide use of private datacom fiber media converters, which are specified in the ElectroniCast market study, is forecast to peak at $1.646 billion in 2017, before slipping to $1.628 billion in 2018.

“The fiber media converters researched in this market study are typically used within an existing Private Enterprise Data Centers (DCs) and Local Area Networks (LANs), as well as other non-public data communication links. They are often used to connect newer 100-Mbps, Gigabit Ethernet, 10G, or other equipment in existing networks, which are generally (copper-based) 10BASE-T, 100BASE-T, or a mixture of both,” stated Stephen Montgomery, Director of the Fiber Optics Components group at ElectroniCast Consultants.

“Several factors make the conversion from copper to optical fiber a good choice, such as – longer link lengths in campuses and industrial plants; resistance to electromagnetic and radio-frequency interference (EMI/RFI) may be necessary; and wider bandwidth capability, just to point-out a few examples,” Montgomery added.

The strong user demand for greater bandwidth and increased interconnectivity to the desktop, throughout the buildings, campuses, from LAN-to-LAN (Metropolitan Area Network – MAN) continues in 2014.

This is matched by rapidly growing demand for global broadband interconnectivity. Interactive multimedia terminals, triple play (voice, video and data), quadruple-play (adding mobility as a communications function to the network), and numerous other dynamics/ applications, continuing bring rapid access to massive databases, which increase productivity while providing rapid ROI (return on investment).

Such expanded capability, however, must often be obtained without making the current network elements obsolete. Local area network (LAN) applications illustrate this trend. LANs are becoming larger and more complex. Reconfiguration, relocation, and extension of LANs are occurring more frequently, due to organization restructuring, advances in computer usage, and the trend toward decentralized computing.

These changes to LAN cabling represent a major ongoing operational expense and a disruption of work for many companies (enterprises). For example, adding capabilities often requires that network administrators upgrade their existing LANs to another media type: for example, copper-to-fiber, multimode-to-singlemode fiber, or even singlemode –to- different types of singlemode optical fiber (note: copper-to-copper conversion is not covered in the study). By using media converters, the network administrator can achieve these upgrades inexpensively.

According to ElectroniCast, the global use of fiber media converters in private datacom reached $1.07 billion in 2013 and is forecast to peak at $1.646 billion in 2017, before slipping to $1.628 billion in 2018.

Private Datacom Fiber Media Converter Global Market Forecast,
(Value Basis, $ Million) – Source: ElectroniCast Consultants

Note: Market forecast data in this study report refers to consumption (use) for a particular calendar year; therefore, this data is not cumulative data.

DK Photonics – www.dkphotonics.com specializes in designing and manufacturing of high quality optical passive components mainly for telecommunication, fiber sensor and fiber laser applications,such as PLC Splitter, WDM, FWDM, CWDM, DWDM, OADM,Optical Circulator, Isolator, PM Circulator, PM Isolator, Fused Coupler, Fused WDM, Collimator, Optical Switch and Polarization Maintaining Components, Pump Combiner, High power isolator, Patch Cord and all kinds of connectors.

Where can WDM-PON go next? — DK Photonics

Where can WDM-PON go next?

The current generation of commercial WDM-PON/ 100GHz DWDM systems based on reflective ONU technology is optimized for applications up to 20 km, 40 channels, and 1 Gbps per customer. Current research focuses on how to scale WDM-PON toward higher bit rates and longer reach. Forward error correction is a key technology for scaling the current generation of WDM-PON technology to higher bit rates, longer reach, tighter channel spacing, or a combination thereof. An important challenge is to package the technology in an MSA form-factor pluggable module to maintain its benefits in cost and compatibility with third-party equipment.

A typical requirement for next generation metro/access systems is to enable node consolidation. That means operators can reduce opex by closing down portions of their central offices; at the same time, this goal requires the optical signals to bridge longer distances than what is typical of the access networks of today. Thus, when routing WDM-PON / 1064nm high power isolator signals through the metro part of the network, it becomes necessary to support ring architectures as an alternative to the basic tree structure.

In a ring structure, cascaded filters may decrease the effective channel passband. Since the spectral width of the WDM-PON signal is wider than the signals from a normal DFB source, such filtering effects may affect transmission.

In a recent evaluation project, a partnership between Transmode and Deutsche Telekom Hochschule für Telekommunikation of Leipzig, Germany, achieved 140-km long reach WDM-PON transmission over a ring-based access-network architecture. The partnership investigated the effects of using WDM-PON based on ASE-seeded wavelength-locked transmitters in a ring-based network architecture with cascaded CWDM OADM nodes. Transmission at 1.25 Gbps over 140-km singlemode fiber was demonstrated using an EDFA and dispersion compensation.

The results were first published at ECOC 2013 (In de Betou, Bunge, Åhlfeldt, and Olson, “140km Long-reach WDM-PON Test for Ring-based Access Network Architecture”). This partnership has investigated what opportunities could be provided by WDM-PON technology in such network topologies by studying experimentally the influence of narrow filtering and maximum reach.

The experimental testbed (in Leipzig) was built around Transmode’s TM-Series iWDM-PON system to create an optical line terminal (OLT) (see Figure 2). The OLT has a transponder line card that hosts pluggable wavelength-locked Fabry-Perot transceivers, ASE seed light sources, dual circulators for up- and downstream, and a 40-channel multiplexer based on an AWG.

To reach distances beyond 100 km, amplifiers dispersion compensation, and remote ASE seed sources were used. While an experimental field trial today, it shows that WDM-PON may well continue to evolve to support longer reach and more sophisticated network architectures in the future supporting a broader range of deployment scenarios.

DK Photonics – www.dkphotonics.com specializes in designing and manufacturing of high quality optical passive components mainly for telecommunication, fiber sensor and fiber laser applications,such as High Power Isolator,1064nm Components,PM Components,Pump Combiner,Pump Laser Protector,which using for fiber laser applications.Also have Mini-size CWDM, Optical Circulator, PM Circulator,PM Isolator, Fused Coupler,Mini Size Fused WDM.More information,please contact us.

WDM-PON technology-DK Photonics

WDM-PON provides the dedicated bandwidth of a point-to-point network and the fiber sharing inherent in PONs. The architecture is somewhat similar to that of EPON and GPON; instead of the power-splitter approach used in TDM-PON architectures, WDM-PON uses an arrayed waveguide grating (AWG) filter that separates the wavelengths for individual delivery to the subscriber ONUs (see Figure 1).

A simple, plug-and-play implementation is based on wavelength-locked or tunable lasers. Self-tuning “colorless” ONUs can be used at the subscriber sites to simplify inventory and spare-part handling. Colorless optics not only simplify operations, but also reduce deployment costs, since they don’t need the expensive wavelength-stability components that traditional fixed and tunable optics require. There are multiple approaches to the colorless ONU technology.

In one approach, the wavelength of the ONU transmitter is controlled by injection of a “seed” signal into the transmitter (e.g., a wavelength-locked Fabry-Perot laser or reflective semiconductor optical amplifier). The seed signal injected into the transmitter could come from broadband ASE light sliced through the filters in the system or from a DFB laser array. In a self-seeding version of this approach, the seed light is provided by feedback of broadband light from the transmitter itself. The passive filtering of the seed light in the remote node determines the wavelength of the ONU transmitter.

In a different approach, the colorless ONU contains a singlemode optic coupler wavelength-tunable laser, which is able to tune to the appropriate wavelength that matches the remote node filter port.

Below 10-Gbps channel bit rates, the injection-seeded method provides a cost-efficient approach. As an example, a wavelength-locked Fabry-Perot transmitter can be integrated into an MSA SFP pluggable form-factor module, which enables the use of third-party CPE devices. A modified EDFA gain block in a 70×90 MSA form factor could be used to generate the broadband ASE light that’s used as a seed signal in the system.

At 10-Gbps bit rates, tunable-laser technology offers an alternative to the injection-seeded approach. The tunable-laser technology developed for the metro/long-haul market has matured significantly over the past couple of years and is able to give a good cost-per-bit ratio when high capacity is needed.

If the WDM-PON system is properly designed, then it’s possible to mix different transmission technologies. By following certain design rules during the installation of the WDM-PON system, it’s possible to allow step-wise channel upgrades to higher bit rates when the demand arises. These design rules ensure that channel OSNR requirements will be met in the presence of reflections and that inter-channel crosstalk is avoided. The result is an open and flexible access network that can support many applications and services over the same infrastructure. WDM-PON thus becomes an optical option for the access network as and where it makes sense.

Given its ability to help service providers cope with current bandwidth demands as well as the next potential broadband access bottleneck, WDM-PON/ 100GHz DWDM Module is becoming an important technology to consider in terms of its benefits and market timing. As with any emerging technology, service providers need to consider the optimal strategy for initial deployment of WDM-PON. That includes how they could use WDM-PON for additional network applications as the technology matures and its costs come down.

WDM-PON technology

FIGURE 2. Architectural scenario explored in the collaboration between Transmode and Deutsche Telekom Hochschule für Telekommunikation.

The latest generations of WDM-PON systems are now gaining traction with operators around the globe for field deployment, lab trials, and evaluations. It’s clearly the early stage of WDM-PON deployments, but progress has started and 2014 looks to be a pivotal year for the technology.

62.5/125 um Vs. 50/125um Multimode fiber Information

We have created this page to illustrate the very basic differences between 62.5 and 50/125 multimode fiber in selecting a patch cable for your existing cable plant.

The key thing to remember is to always use a patch cable of the same type as the cable that you are connecting to. It is virtually impossible to tell the difference between the two fiber types (62.5 and 50/125) by looking at the bare fiber* or the connectors*. Usually, this information will be written on the cable’s jacket.

The photos above illustrate that the outer diameters of the two fiber types are the same. What is different is the size of the center light carrying core of the fiber. You cannot see the fiber’s core without a microscope*. Therefore, you must rely on the writing that is on the fibers jacket to determine what type is.

Severe losses of light can occur when you try to match 50/125 and 62.5/125 fiber, as the illustration on the left shows.

* CAUTION: Never look directly into a fiber cable’s end face or into the ferrule of a connector (with fiber present) as there may be dangerous laser light present.

NOTE: This page was designed to help you know the difference between 62.5 and 50/125 fiber for the purpose of purchasing patch cables and products to connect to existing installed cabling. This page was not designed to provide information on choosing between the two types fiber for new installations.

Pump and signal combiner for bi-directional pumping of all-fiber lasers and amplifiers(5)

4.3 Simulations for the loss mechanism of the fiber combiner

As already discussed in Section 2, the total 1064nm high power isolator loss is the sum of TP, PAA and PCT (Fig. 1). In this section we will quantitatively determine the power fraction of the different loss mechanisms to gain a better estimate of the resulting thermal load of the fiber combiner. To understand this approach, we first discuss the effect of the different loss mechanisms. The TP pump power loss is less critical, because this power fraction can be easily removed from the fiber component via the IF. The PAA is also less critical, since this power fraction can be handled by an air or 100W 1064nm high power isolator housing. The most critical pump power loss, PCT, is caused by NA-mismatched light, which couples into the coating of the TF and damages the fiber coating at a certain power level.

Fig. 4 The loss mechanism and the total pump power loss of the fiber combiner for (a) a TL of 5 mm and (b) a TL of 20 mm at different taper ratios. The losses in percent were calculated with respect to the total input pump power. Please see Fig. 1 for TP, PCT and PAA.

and 4(b) shows the three different pump power losses (TP, PAA, PCT) and the total pump power loss as a percentage of the input pump power for TL of 5 and 20 mm, depending on the TR. In the simulations the core NA of the PFF was 0.22 and fully filled pump light condition of the PFF core was assumed. It should be noted that for comparison, the axis of ordinates in Figs. 4(a) and 4(b)are scaled differently for a more comprehensive presentation of the results. In general, it can be seen that the total and individual losses are larger for a TL of 5 mm compared to a TL of 20 mm. For both TLs it turns out that the TP-fraction decreases and the PCT-fraction as well as the PAA-fraction increases with TR. As a result, the total power loss decreases with increasing TR. A closer analysis of the PCT-curve reveals that PCT loss does not exist below a TR of 2, since the 3 Port Polarization Maintaining Optical Circulator input NA of 0.22 will be approximately increased by the factor of the TR [18], and therefore cannot exceed the cladding NA of the TF of 0.46. Thus, the fraction of PCT can be reduced by choosing a low TR with a still acceptable total power loss. This means that the TR must be carefully adapted to satisfy the trade-off between a high pump coupling efficiency and a low power fraction of PCT to avoid optically induced damage of the fiber component during high power operation. This must always be accompanied by a sufficient converging taper length.

For example, if the TR is set to 7 for a TL of 5 and 20 mm, respectively, the theoretical PCT is 7.7 and 1.2% of the input pump power. The PCT value of 1.2% at a TL of 20 mm can be further reduced to 0.6% by changing the TR from 7 to 4 in conjunction with an acceptable total power loss of just 5%. Hence, if 1 kW of input pump power is assumed, the resulting power handling for the coating of the TF and the pump light stripper can be reduced from 77 W (TL 5 mm, TR 6) to 6 W (TL 20 mm, TR 4) by adapting the TL and the TR.

The simulations indicate that the minimum total power loss cannot be reduced below 2.7% for a TL greater than 20 mm up to a TL of 50 mm and a FL of 1.99. One reason for the residual losses can be pump light rays with a Polarization Maintaining Fused Coupler, which propagate along an unfavorable plane of the IF and do not enter the fusion zone. These rays leave the waveguide (PAA) structure after sufficient bounces along the lateral taper surface. In addition, rays with an extremely low NA, and consequently less bounces with the lateral surface of the converging taper portion, can occur in the form of TP. Furthermore, longer TLs lead to an increased probability that some rays will reverse couple from the TF into the IF.

Moreover, the simulations reveal that a lower FL-value, which means stronger fusing of the fibers, leads to a decrease of the total power loss. The exact reduction of the total power loss depends on the fiber and taper parameters. For a TL of 20 mm and a TR of 6, the simulated total power losses could be reduced from 4% to 2% when decreasing the FL from 1.99 to 1.93. The simulations indicate that for FLs below 1.93 the total power loss increase again.

4.3.1 Impact of pump light input NA on the power leakage into the coating of the TF (PCT)

The simulations in Section 4.2, Fig. 3(b) showed that a sufficient TL leads to pump coupling efficiencies of more than 90%, almost independent of the pump light input NA. Considering the losses, the simulation also shows that the PCT-fraction is strongly influenced by the pump light input NA. Figure 5

Fig. 5

The ratio of power leakage into the cladding of the target fiber

(PCT) to the total input pump power against the taper ratio for a TL of 20 mm.

clearly reveals that for a TL of 20 mm and a TR of 6, the PCT-fraction increases by about 6 times for a NA of 0.3 compared to a NA of 0.15. Hence, it is possible to achieve almost the same coupling efficiency for a pump light input NA of 0.15 and 0.3 (see Fig. 3(b)), but with a significant difference in risk of optically induced damage to the fiber component. However, PCT can be further reduced by increasing the TL.

Pump and signal combiner for bi-directional pumping of all-fiber lasers and amplifiers(4)

4. Simulations and experiments for a fiber combiner with a single pump port

The ray tracing simulations were carried out with the commercially available software Zemax (Radiant Zemax, LLC) in the non-sequential mode. Detailed information about ray tracing in tapered cylindrical fibers can be found in Ref [16] and [17]. The ray tracing method is applicable due to the large cross sections of the employed fibers compared to the applied wavelength of 976 nm. The 3-dimensional simulation model of the fiber combiner was based on the setup depicted in Fig. 1 with the approximation of a parallel fiber arrangement of the IF and TF. For the PFF a fully filled condition was always assumed, meaning that all possible pump light rays, independent of the NA and the transversal position in the fiber core, carry equal power. For the geometrical shape of the taper in the longitudinal direction, a simplified linear shape was assumed in the simulations, instead of the measured parabolic shape. As already mentioned, the FL was set to 1.99.

4.1 Simulations of the pump coupling efficiency

The pump coupling efficiency in dependence of the converging taper length (TL) and the taper ratio (TR) of the IF for a 1064nm high power isolator with an NA of 0.22 is depicted in Fig. 2(a)

Fig. 2 (a) Pump coupling efficiency (CE) with respect to the taper ratio (TR) and the converging taper length (TL) and (b) a comparison of the pump coupling efficiencies without intermediate fiber (IF) and with IF for different fiber parameters, IF Ø: IF cladding diameter.

. The simulations show that an increasing TL leads to higher coupling efficiencies at a constant TR. For example at a constant TR of 6 a TL of 5 mm leads to a theoretical maximum pump coupler coupling efficiency of 86%, whereas for a TL of 20 mm 96.4% were calculated. Furthermore, Fig. 2(a) shows that the TR can be reduced, if the TL is increased to maintain a certain coupling efficiency level. For instance, for a TL of 20 mm, a coupling efficiency of 85% can already be obtained at a TR of 2 instead of a TR of 5.5 at a TL of 5 mm. The improved coupling behavior at longer TLs can be explained by the increasing number of bounces of the pump light rays at the lateral surface of the converging taper portion. Hence, for shorter TLs it is necessary to taper more than for longer TLs in order to compensate for the shorter interaction length of the converging taper portion with the TF. The maximum theoretically obtainable pump coupling efficiency was limited to 97.3% due to different loss mechanisms, which will be discussed in Section 4.3.

In the following section we discuss the impact of the intermediate fiber on the pump coupling efficiency and the taper parameters. Thus, for comparison the fiber combiner was also simulated without the IF, which means that the tapered PFF was directly connected to the TF, assuming the same FL and also a NA of 0.22. Figure 2(b) illustrates that the coupling efficiency can be increased and the TR reduced, if an IF is inserted between the PFF and the TF. For a TR of 2.5 at a TL of 20 mm the coupling efficiencies with and without IF are 61.2% and 90.1%, respectively. The moderate coupling efficiencies without the employment of an IF at low TR can be explained by the presence of a depressed refractive index of the cladding of the PFF, blocking the power transfer from the IF to the TF, as already discussed in Section 2. Thus, without IF, the pump light rays with a low NA cannot escape from the core of the PFF, and a considerable fraction of power will be transmitted via the diverging taper portion. A further increase of the pump light NA, due to the increase of the TR up to 10 at a TL of 20 mm for the PFF and the IF, results in a successive approximation of the Polarization Maintaining Optical Circulator efficiencies. However, even at a TR of 10 and a TL of 20 mm (with IF) a 2.5% higher pump coupling efficiency can be obtained. That means for a hypothetical available input pump power of 1 kW, a reduction in power loss of 25 W can be essential to prevent thermal damage of the fiber combiner. Additionally, it must be taken into account that a TR of 10 corresponds to a considerable reduction of the mechanical stability due to the fiber diameter tapering from 125 µm to 25 µm. Furthermore, Fig. 2(b) clearly shows that the insertion of an IF with a TL of 10 mm already yields better pump coupling efficiencies than a PFF with a TL of 20 mm, especially for low TR.

A further increase of the pump coupling efficiency up to 97.8% can be realized by inserting an IF with a TL of 20 mm and diameter of 105 µm, which is perfectly adapted to the core diameter of the PFF, and thus, no pump brightness loss occurs. Note that for all of the following simulations and experiments, we only used the fiber component containing an inserted IF with a cladding diameter of 125 µm.

4.2 Simulations for the impact of the pump light input NA on the pump coupling efficiency

In the next simulation step we figure out, how the pump coupling efficiency changes with the pump light input NA depending on TR and TL. For these simulations three types of PFFs with a core NA of 0.15, 0.22 and 0.30 were investigated, assuming for each PFF a fully filled pump light condition. The TR was considered in the range from 1 to 10 at a TL of 5 mm

Fig. 3 Pump coupling efficiency with respect to the taper ratio at a converging taper length of (a) 5 mm and (b) 20 mm for a PFF with a pump light input NA of 0.15, 0.22 and 0.30.

) and 20 mm (Fig. 3(b)). From both figures it can be seen that at lower TRs the coupling efficiency increases with NA, since the pump light rays with a higher NA have more bounces with the lateral surface of the converging taper portion. However, the pump coupling behavior changes with increasing TR, since a TR of much higher than 2 leads to pump light rays with a NA far above 0.46, which cannot couple into the TF, if the TL is too short. The occurring pump power losses will be discussed in Section 4.3. E.g., for a low TL of 5 mm and a TR of 7 the coupling efficiency for an input NA of 0.15 was simulated to be 10% higher than for an input NA of 0.30. In contrast, with a longer TL of 20 mm the coupling efficiency seems to be less sensitive to variations of the pump light input NA. Thus, it appears that for the combiner design, the pump coupling efficiency should not be significantly influenced by the pump light input NA in the range of 0.15 to 0.30, if a sufficient TL is considered.

If the pump light input NA gets closer to the NA of the TF of 0.46, it can be advantageous to use a straight IF portion in addition to the converging taper to obtain a highly efficient pump light transfer into the TF as described in Ref [13]. An alternative approach to the straight IF portion is an increased TL, i.e. for a pump light input NA of 0.46 a theoretical pump coupling efficiency of about 90% can be achieved, if the TL is at least 40 mm.

Pump and signal combiner for bi-directional pumping of all-fiber lasers and amplifiers(3)

3. Fabrication

The IF was fusion spliced to the DK Photonics with a filament splicing system (Vytran FFS-2000). A hydrogen-oxygen micro-flame was applied as heat source for tapering and lateral splicing of the IF. The working temperature for the tapering as well as the weak lateral splicing process of the IF was not measured but it can be assumed to be between the annealing and softening point of fused fiber coupler. The temperature adjusting was controlled by variation of the vertical distance between the fiber and the flame. Two precisely controlled motor stages were used to allow accurate alignment and tapering of the fiber(s). The heat source was placed at a fixed position in the center between the two motor stages. Each IF was individually tapered with a pulling speed of about 40 µm/s per motor stage and a fiber tension of about 10⁻² N. After tapering, the IF was once twisted around the TF, which ensures that the converging taper portion remain in contact during lateral fusing. In case of a fiber combiner with several pump ports (see Section 5), the IFs were also individually tapered but simultaneously twisted around the TF. The final lateral fusion process along the converging taper portion was carried out at temperatures which allow sufficient softening of the tapered IF(s) and only slightly softening of the TF resulting in a weak fused component without any thermally induced damage of the core of the TF.

4. Simulations and experiments for a fiber combiner with a single pump port

The ray tracing simulations were carried out with the commercially available software Zemax (Radiant Zemax, LLC) in the non-sequential mode. Detailed information about ray tracing in tapered cylindrical fibers can be found in Ref [16] and [17]. The ray tracing method is applicable due to the large cross sections of the employed fibers compared to the applied wavelength of 976 nm. The 3-dimensional simulation model of the fiber combiner was based on the setup depicted in Fig. 1 with the approximation of a parallel fiber arrangement of the IF and TF. For the PFF a fully filled condition was always assumed, meaning that all possible pump light rays, independent of the NA and the transversal position in the fiber core, carry equal power pump combiner. For the geometrical shape of the taper in the longitudinal direction, a simplified linear shape was assumed in the simulations, instead of the measured parabolic shape. As already mentioned, the FL was set to 1.99. Table 1 shows a summary of the fiber parameters used for simulations:

shows a summary of the fiber parameters used for simulations: — shows a summary of the fiber parameters used for simulations