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Polarization Dependent Isolator vs Polarization Independent Isolator

Connectors and other types of optical devices on the output of the transmitter may cause reflection, absorption, or scattering of the optical signal. These effects on the light beam may cause light energy to be reflected back at the source and interfere with source operation. In order to reduce the effects of the interference, an optical isolator is usually used. Optical isolator allows a beam of light to stream through a single one way direction. At the same time, it prevents the light from going back in the opposite direction. According to the polarization characteristics, optical isolators can be divided into two types, including polarization dependent isolator and polarization independent isolator. The polarizer-based module makes a polarization dependent isolator, and the birefringent crystal-based structure makes a polarization independent isolator. You may be very confused about them as you find that there is only a little difference via their names. So, what are they and what are the differences between them? This paper will give you the answer.

Polarization Dependent Isolator

The polarization dependent isolator consists of three parts, an input polarizer , a Faraday rotator, and an output polarizer. Light traveling in the forward direction becomes polarized vertically by the input polarizer. The Faraday rotator will rotate the polarization by 45°. The analyser then enables the light to be transmitted through the isolator.

Light traveling in the backward direction becomes polarized at 45° by the analyser. The Faraday rotator will again rotate the polarization by 45°. This means the light is polarized horizontally. Since the polarizer is vertically aligned, the light will be extinguished.

The picture shows us a Faraday rotator with an input polarizer, and an output analyser. For a polarization dependent isolator, the angle between the polarizer and the analyser, is set to 45°. The Faraday rotator is chosen to give a 45° rotation.

Because the polarization of the source is typically maintained by the system, polarization dependent isolator is widely used in free space optical systems.

Polarization Independent Isolator

The polarization independent isolator also consists of three parts, an input birefringent wedge, a Faraday rotator, and an output birefringent wedge. Light traveling in the forward direction is split by the input birefringent wedge into its vertical (0°) and horizontal (90°) components, called the ordinary ray (o-ray) and the extraordinary ray (e-ray) respectively. The Faraday rotator rotates both the o-ray and e-ray by 45°. This means the o-ray is now at 45°, and the e-ray is at −45°. The output birefringent wedge then recombines the two components.

Light traveling in the backward direction is separated into the o-ray at 45, and the e-ray at −45° by the birefringent wedge. The Faraday Rotator again rotates both the rays by 45°. Now the o-ray is at 90°, and the e-ray is at 0°. Instead of being focused by the second birefringent wedge, the rays diverge. The picture shows the propagation of light through a polarization independent isolator.

While polarization dependent isolator allows only the light polarized in a specific direction, polarization independent isolator transmit all polarized light. So it is usually widely used in optical fiber amplifier.

Comparison of Polarization Dependent Isolator and Polarization Independent Isolator

In fact, you have already understood these two types of isolators according to the contents above. We can see their similarities and differences through the comparison of their definition, working principle and applications. Both of them consist of three parts and have a same principle based on Faraday effect. However, to overcome the limitation of polarization dependent isolator, polarization independent isolator has been developed. Regardless of the polarization state of the input beam, the beam will propagate through the isolator to the output fiber and the reflected beam will be isolated from the optical source. If the extinction ratio is important, a polarization dependent isolator should be used with either polarization maintaining fibers or even regular single-mode fibers. If the system has no polarization dependence, a polarization independent isolator will be the obvious choice.

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DK Photonics – www.dkphotonics.com specializes in designing and manufacturing of high quality optical passive components mainly for fiber laser applications such as 1064nm high power isolator, Cladding Power Stripper, Multimode High Power Isolator, pump combiner,1064nm Band-pass Filter,(6+1)X1 Pump and Signal Combiner, PM Circulator, PM Isolator, optical Coupler. More information, please contact us.

Large Mode Area Fibers

Optical fibers with relatively large mode areas and a single transverse mode or only a few modes.

For some applications, it is desirable to use optical fibers with a large mode area (LMA fibers) – often with single-mode guidance. Due to the reduced optical intensities, such fibers effectively have lower nonlinearities and a higher damage threshold, which makes them suitable for example for the Amplification of intense Pulses or single-frequency signals in Fiber amplifiers, or in case of passive fibers for delivery of such light. While standard single-mode fibers have an Effective Mode Area below 100 μm2, large mode area fibers reach values of hundreds or even thousands of μm2.

Pump and signal combiner for bi-directional pumping of all-fiber lasers and amplifiers(7)

Pump and signal combiner for bi-directional pumping of all-fiber lasers and amplifiers(7)

5. Simulations and results for a multi pump port configuration

So far, the modeling results consider a TF with only a single pump port. However, for monolithic high power fiber laser and amplifier systems, it is often required to provide multiple pump ports due to the limited output power of available fiber coupled pump diodes and the efforts to develop laser systems with redundancy. Thus, in this section, we investigate the impact of multiple pump ports on the coupling efficiency and the loss mechanism. The setup of each pump combiner is identical to the description in Section 2 (see Fig. 1), but with several additional ports placed around the cladding of the TF, leading to a fiber bundle. A schematic of a fiber combiner with multiple pump ports is shown in Fig. 7

Pump and signal combiner for bi-directional pumping of all-fiber lasers and amplifiers(7)

Fig. 7 Fiber combiner with multiple pump ports, PFF: pump feeding fiber with a piece of coreless intermediate fiber (IF) as described in Fig. 1, TF: target fiber, TP: transmitted power.

5.1 Simulations of the pump coupling efficiency

The experiments and simulations in Section 4 showed that for a pump combiner with a single pump port, a TL of 20 mm and a TR of 6 yields an excellent coupling efficiency in the range of 95%. In comparison, for a fiber band pass filter with multiple pump ports, the simulations for a TL of 20 mm (Fig. 8(a)

Pump and signal combiner for bi-directional pumping of all-fiber lasers and amplifiers(7)-2

Fig. 8 Simulated coupling efficiency for a pump combiner with up to 6 pump ports for (a) a TL of 20 mm and (b) a TL of 10 mm for a pump light input NA of 0.22.

) revealed that the pump coupling efficiency of the combined pump power depends on the number of pump ports and significantly on the choice of the TR. In the simulations the input pump light NA of the PFFs was 0.22. In general, it can be seen that the pump coupling efficiency decreases with each additional pump port. A lower TR yields a greater decrease of the pump coupling efficiency with each additional pump port than a higher TR. In the case of a TL of 20 mm and a TR of 2.5, the theoretically obtainable pump coupling efficiency of almost 90% decreases to 73%, if the number of pump ports increases from 1 to 6. However, as already mentioned, the increasing losses due to additional pump ports can be reduced with increasing TR. In Fig. 8(a) it can be clearly observed that for 6 pump ports and a TR of 6, a pump coupling efficiency of 90.2% can be achieved. For a TR higher than 6, it is not possible to achieve a significant improvement in pump coupling efficiency for multiple pump ports by increasing of the TR.

For a single pump port configuration it is already known that the pump coupling efficiency decreases with shorter TLs at constant TRs (Fig. 2(a)). However, for multiple pump ports a reduction of the TL leads to the advantage that the pump coupling efficiency of the combined pump power decreases less with each additional pump port, especially at lower TRs. The simulation results for a TL of 10 mm instead of a TL of 20 mm are presented in Fig. 8(b). A comparison of Fig. 8(a) and 8(b) shows: If the number of pump ports is increased from 1 to 6 at a TR of 2.5, the pump coupling efficiency experiences a decrease of 16.9 and 11.2% for a TL of 20 and 10 mm, respectively. Although the total power losses for a TL of 10 mm are higher than for a TL of 20 mm, the example reveals, that the decrease of the pump coupling efficiency due to additional pump ports can be reduced by using shorter TLs.

Besides having less available combined pump power, the additional pump power losses generated in comparison to a fiber combiner with a single pump port, corresponds to an enhanced risk of damaging the component due to additional thermal load. Hence, the loss mechanism for a fiber combiner with multiple pump ports needs to be investigated in more detail.

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Optical Filters: Filter stacks transmit wide-angle incident light without shifting wavelength(3)

To avoid the problem of color change versus incidence angle in an optical system, thin-film-coated filter elements can be replaced by a filter consisting of a stack of different filter glasses.

JASON KECK

Rugged, no coating degradation

Advantages of using a filter stack rather than a thin-film-coated optical element include wide-angle performance (see Fig. 2) and high durability. Because the glass itself performs the blocking, there is no concern of coating degradation due to extreme environmental shifts, contamination, or mishandling. Filter stacks are as durable as the glass they are made from, surviving aggressive cleaning methods, severe abrasion, salt/fog testing, humidity, and temperature cycling per durability standards of MIL-PRF-13830B, MIL-C-48497A, and MIL-C-675C.

Because all filter glass types have approximately the same index of refraction, there is no Fresnel loss as light propagates from one internal layer to another. However, as with any glass, the air-to-substrate interfaces will incur an ~8% total Fresnel loss for the component.

The addition of a broadband antireflection (BBAR) coating on each air-to-substrate surface can mostly eliminate this loss. The spectral range of the BBAR is designed to be much wider than the active spectral region of the 100G DWDM filter, so the stability of the transmission band will not be affected by changes in the angle of the filter. Blocking coatings can also be added if it is necessary to create steeper edges for in-band performance; however, doing so can affect the wide-angle performance at the edge wavelengths.

ColorLock filter stacks can be designed for spectral ranges from ultraviolet to near-infrared, with transmission exceeding 60% at the specified design wavelength. This transmission may not be as high as with dielectric filters, but is sufficient for applications with controlled and stable illumination, such as for machine vision, in which the consistency of wavelengths from wider incident angles is more important than transmission.

Having overcome considerable design challenges, we believe that these filter stacks can be used as an innovative solution in applications that demand consistent wavelengths from incident angles that are wide enough that dielectric filters would not be sufficient, and where the higher transmission that is afforded by dielectric filters is less important.

Industrial Fiber Laser Introduction and Global Market Forecast –DK Photonics

The Global Industrial Fiber Laser market to grow at a CAGR of 21.4% over the period 2013-2018

Fiber lasers contain the active gain medium, which is an optical fiber integrated with rare earth elements such as erbium and ytterbium. Unlike conventional gas lasers, a fiber laser uses part of the fiber as the resonating cavity, where the laser action takes place to generate laser beams , Fiber lasers are preferred over other lasers such as CO2 lasers and excimer lasers, primarily because they are more reliable, efficient, robust, and portable, and easier to operate than other lasers.

Fiber lasers used for industrial applications such as cutting, welding, marking, and engraving in the Manufacturing, Semiconductor, and Automotive industries are referred to as industrial fiber lasers. Moreover, due to their superior performance, compact size, high output power, low cost of ownership, durability, and eco-friendly attributes, industrial fiber lasers are being adopted at a significant rate. They also eliminate the mechanical adjustments and high maintenance costs that are necessary with other lasers.

Increased R&D spending by vendors to gain a competitive advantage over other players in the market is one key trend in this market. Vendors are increasingly investing in their R&D division to provide better functionality and to meet the unsatisfied requirements of consumers. R&D investments have enabled vendors to capture a significant market share and gain a competitive edge over other vendors in the Global Industrial Fiber Laser market.

According to the report, one major driver of the market is the increased adoption of fiber lasers because of their superior attributes. These lasers used for industrial applications are gaining more significance because they exhibit excellent light properties.

Further, the report states that one of the key challenges that the market faces is the uncertainty regarding the lifespan of fiber lasers. Despite their existence in the industry for more than 10 years, the lifespan fiber lasers are not definite.

Pump and signal combiner for bi-directional pumping of all-fiber lasers and amplifiers(1)

Abstract

We developed an all-fiber component with a signal feedthrough capable of combining up to 6 fiber-coupled multi-mode pump sources to a maximum pump power of 400 W at efficiencies in the range of 89 to 95%, providing the possibility of transmitting a high power signal in forward and in reverse direction. Hence, the fiber pump combiner can be implemented in almost any fiber laser or amplifier architecture. The complete optical design of the combiner was developed based on ray tracing simulations and confirmed by experimental results.

1. Introduction

For the realization of compact, reliable, rugged and efficient monolithic high power fiber laser systems, the efforts of integrating all-fiber components have been increased in recent years [1,2]. A key component of a highly integrated fiber laser or amplifier system is a high power all-fiber signal and pump combiner.

The most common type of fiber combiner, a fused tapered fiber bundle (TFB) [3,4], is based on the fiber end face pumping technique and is probably the most sophisticated pump combiner capable of handling several hundred watts of pump power [5]. A TFB with signal feedthrough consists of a central input signal fiber, guiding the signal light, surrounded by several multi-mode fibers, guiding the pump light, and an output pigtail double-clad (DC) fiber which combines the signal and pump light in a single pigtail fiber. In order to match the diameter of the fiber bundle to the diameter of the output pigtail fiber, the bundle is slowly melted and tapered. After the tapering process the fiber bundle is cleaved around the taper waist and fusion spliced to the output pigtail DC fiber. However, tapering of the fiber bundle inherently involves increasing the numerical aperture (NA) of the pump light and a change of the mode field diameter (MFD) of the signal light. Hence, the necessary optical matching and mechanical alignment requirements between the tapered fiber bundle and the output pigtail DC fiber can lead to several drawbacks of the TFB structure: (1) less flexibility in the choice of input fibers that match the output pigtail DC fiber after the tapering process, (2) a slight mismatch or misalignment between the signal mode field diameters (MFD) of the tapered input signal fiber and the output pigtail DC fiber leads to a degradation of the beam quality, primarily in conjunction with signal insertion loss, and (3) in the case of a backward propagating signal, e.g. for a counter-propagation pumped fiber amplifier, the signal insertion loss (up to 10%) can cause damage to the pump diodes due to their insufficient isolation against amplified signal light.

A more promising approach to overcome these problems is side-pumping technology, which involves coupling the pump light via the outermost cladding surface into the fiber. The key advantage of this technology is the uninterrupted signal core, eliminating the need for an additional fusion splice in conjunction with signal mode matching. In recent years several proposals for side-pumping of DC fibers have been reported, such as V-groove side pumping [6], a mirror embedded in the inner cladding of a DC fiber [7] or side-coupling by an angle polished pump fiber [8]. However, for most of these side-pumping configurations it is difficult to reach the mechanical accuracy required for a stable and efficient pump light coupling.

A more rugged approach is a monolithic all-fiber combiner like the GT-Wave coupler [9], the employment of a tapered capillary around a multi-clad fiber [10, 11] or direct fusion of one or more tapered multi-mode fibers to the outermost cladding of multi-clad fibers [12–14]. In Ref [11] seven pump delivery fibers with a core diameter of 110 µm (NA 0.22) were combined and laterally coupled via a tapered capillary into a DC fiber with a core diameter of 400 µm (NA 0.46), which led to a combined pump power of 86 W with a coupling efficiency of ~80%. In Ref [13], direct lateral fusion of one tapered pump delivery fiber with a core diameter of 200 µm (NA 0.46) to a DC fiber of 250 µm (NA 0.46) led to a coupling efficiency of 90% at a pump input power of 120 W, furthermore, a pump delivery fiber with a diameter of 400 µm (NA 0.46) was used to couple a pump power of 300 W with an efficiency of 85% into a DC fiber with a diameter of 400 µm (NA 0.46). These impressive coupling efficiencies for one pump port were achieved by use of a straight and a tapered fiber section, allowing for highly efficient coupling of pump light rays with a high numerical aperture. Thus, in Ref [13] the impact of the straight fiber section on the side-pump coupling process was discussed. However, a review of the literature reveals that the impact of the fiber and taper parameters on the pump coupling behavior as well as the loss mechanism have not yet been investigated in detail for side-pumped combiners based on direct fusion of one or several tapered multi-mode fibers to the outermost cladding of a DC fiber.

We report detailed simulations and experiments for a component which combines up to 6 multi-mode fibers with a core diameter of 105 µm (NA 0.15 or 0.22) into a DC fiber with a cladding diameter of 250 µm (NA 0.46) via side-coupling. Firstly, we explain the principle of the optical design of the fiber combiner. For a fiber combiner with a single pump port, the achievable pump coupling efficiency and the corresponding loss mechanisms were investigated. For multiple pump ports, the simulations and experiments showed that with each additional pump port, the taper parameters need to be adjusted in comparison to a single pump port configuration. These simulation results can also be used as an estimation for fiber combiners, which combine one or several multi-mode fibers with a core diameter of 200 µm (NA 0.22) into a DC fiber with a cladding diameter of 400 µm (NA 0.46). Therefore, this work covers two important fiber combiner types, since active fibers with cladding diameters of 250 or 400 µm are typical sizes provided by fiber manufacturers and used for continuous wave and pulsed laser systems. In addition, we also investigated the signal feedthrough of the combiner. We demonstrated a low signal insertion loss, maintenance of an excellent signal beam quality and an efficient isolation of the pump diodes against signal light in the case of a reverse propagating signal. The preservation of the signal light properties by the fiber combiner was utilized in Ref [15] for the realization of a counter-propagation pumped single-frequency fiber amplifier with an amplified signal power of 300 W.