1. Introduction

The power of diffraction-limited fiber-lasers in the master-oscillator power-amplifier (MOPA) configuration can be scaled into a high energy laser (HEL) by combining multiple units using coherent or spectral beam combining [1]. For practical applications, fiber MOPAs are optically pumped by fiber-coupled laser-diode modules (FCMs).

The size, weight, and power (SWaP) of the MOPA is mainly determined by the SWaP of the pump modules. Low SWaP is required for deployment of the HEL on a mobile platform, e.g. aircraft and truck [2].

Once the pump power is achieved, the most critical parameter of the FCMs is the power conversion efficiency (PCE) which determines the size of the electrical power supply and the cooling system [2]. While the size and volume parameters are system defined, it is clear that the PCE of a low-SWaP module should be at least 50% where half the electrical power is converted to useful optical power and the other half is converted to waste heat that must be removed by the cooling system.

Presently, in the most developed fiber MOPAs, the gain fiber is a Yb-doped, large mode-area double-clad fiber. Pump power is introduced into the 400µm, 0.46 first clad using a (6 + 1):1 multi-mode fiber combiner in which the signal fiber is undisturbed [3]. For this configuration, the core diameter and numerical aperture of the pump delivery fibers have maximum values of 225µm and 0.22, respectively.

To date, ~600W is the highest reported power for high efficiency FCMs with a 225µm, 0.22NA delivery fiber [4–6]. Single mode fiber amplifiers pumped with these FCMs is able to achieve ~3kW. However, there are already reports of single mode fiber amplifiers achieving 5kW output [7,8]. The 900W pump modules [7,8] have a 200µm, 0.22NA delivery fiber but details, e.g. PCE and laser-diode sources, have not been reported.

In this study, we report on the first, 1000W, fiber-coupled module with 50% PCE to optically pump fiber-amplifiers. The 1000W mode-stripped output from a detachable 225µm, 0.22NA delivery fiber is achieved by combining 10 laser-diode bars at a single wavelength of 976nm. The FWHM spectral width at 1000W output is ~4nm, an excellent overlap with the narrow absorption spectrum of ytterbium in glass. Pump power with this narrow spectral width has been demonstrated to result in more than 89% optical-to-optical conversion efficiency [4] in a Yb-doped fiber amplifier.

2. Module design

All high-brightness, fiber-coupled, multi-mode diode-laser modules have a similar design. The differences depend on the application and the desired operating characteristics of the module. The generic module design is as follows.

The WL1000 fiber-coupled module in this report has two desirable operating characteristics, i.e. 1000W power from a 225µm, 0.22NA delivery fiber and a minimum PCE of 50%.

The PCE of the module is the product of the PCE of the laser diode sources and the fiber coupling efficiency. For the WL1000, the laser-diode (LD) sources are 19-emitter, 20% fill-factor LD-bars emitting at ~976nm. Since high power and high coupling efficiency are desired, LD-bars, which have the most compact configuration of laser-diode emitters, are selected. Since high module PCE is desired, the LD-bars must be cooled. The LD-bars are directly attached to a custom, low thermal-impedance, copper micro-channel cooler (MCC) [9]. The thermal impedance of 4mm cavity length bars in this configuration is determined to be ~0.2°C/W [9].

The light output and efficiency as a function of operating current of a typical LD-bar is shown in Fig. 1. The PCE at 130A, the intended operating current is ~66%, close to the peak efficiency of ~67% at 80A. The small decrease in PCE with operating current is indicative of the low thermal impedance of the heatsink.

 

Fig. 1 Output power and power conversion efficiency (PCE) as a function of operating current of a typical laser diode bar directly attached to a micro-channel cooler.

Download Full Size | PDF

Direct die attach onto the MCC results in the lowest thermal resistance for a LD-bar. Despite the known superior performance of a LD-bar attached directly to a MCC, this technology has been avoided for two reasons. Due to the difference in coefficient of thermal expansion (CTE) between the copper heatsink (~17ppm/°C) and the GaAs laser diode (~5.6ppm/°C), direct attachment of the LD-bar to the MCC is generally accomplished by soldering with indium. The malleable indium relieves the mechanical stress from the CTE mismatch [10] between the LD and the heatsink. However, the indium solder interacts with the gold from the LD-bar to form a mixture of AuIn₂ particles within an indium matrix. With thermal cycling [10], due to the difference in CTE between the AuIn₂ and indium, the AuIn₂ separates from the indium causing the bond to weaken. This weakening of the bond is undesirable for many applications. Secondly, copper MCCs degrade by electro-corrosion if significant current passes through the coolant. Generally, electro-corrosion is minimized by the use of deionized water whose resistivity, pH, and oxygen content are carefully controlled [11].

For the WL1000, both prior problems have been solved. The LD-bars are attached with an indium diffusion-bond [12] which has been empirically found to be stable. A fiber coupled module (225µm, 0.22NA fiber) containing five LD-bars directly attached to copper MCCs using indium diffusion bonding was subjected to hard-pulse operating conditions of 100A peak current, 500ms-on, 500ms-off. No degradation in the fiber coupled power was observed after more than 1.5 million pulses [13].

In the WL1000, the copper MCCs are widely spaced (~17mm) to reduce electric fields at the coolant inlets and outlets where electro-corrosion can occur. Using carefree distilled water as a coolant, this configuration has been found to empirically reduce electro-corrosion [13,14] to an acceptable level. The lifetime of a copper MCC limited by electro-corrosion was defined as the time required for the pressure coefficient to change by 50%. An increase in pressure coefficient indicates clogging of the micro-channels. A decrease in pressure coefficient indicates corrosion of the micro-channels. A lifetime of more than 50khrs was found for a test module containing four LD-bars operated with a coolant of 40% ethanol-water solution flowing at 1.2LPM at a coolant inlet temperature of 25°C [13]. The LD-bars were operated electrically in series at 80A.

The beam parameter product (BPP), a measure of the optical brightness, can be used to design a FCM [15]. The value of BPP is one half the physical aperture times the numerical aperture and the units are mm-mrad. The BPP of a 225µm, 0.22NA fiber is a circle of 24.75mm-mrad diameter. For efficient coupling into a fiber, the BPP of the combined LD-bars must be smaller than the BPP of the fiber. The bare LD-bars used in this study have a BPP_FA along the fast axis of ~0.75mm-mrad and a BPP_SA along the slow axis of ~60mm-mrad. Clearly, the coupling efficiency into a fiber with BPP of ~25mm-mrad is poor.

While optics cannot reduce the BPP, it can rearrange the BPP [15]. Fortunately, a beam transformation system (BTS) to rearrange the BPP is commercially available [15]. Using a BTS, the BPP_FA of the LD-bar is transformed to ~15mm-mrad whereas the BFF_SA is transformed to ~3mm-mrad. This transformed, rectangular BPP of the LD-bar fits within the circular BPP of the fiber and the coupling efficiency can be very high.

Figure 2 is a top view schematic of the WL1000 module. The ten LD-bars on MCCs, labeled LD-1 through LD-10 are placed vertical to the optical plenum to simplify assembly. A single optical plenum is used for stability. Item 1 is the female SMAQ connector with custom MCC heatsink described below. Item 2 and 3 are the laser diode anode and cathode, respectively. Item 4 is the BTS micro-optics. Items 5, 6, 7, 8, 9 are the slow-axis collimation lenses, the turning mirrors, the turning prisms, the polarization beam combiner, and the aspheric lens, respectively.

 

Fig. 2 Top view of the WL1000 fiber-coupled module. The ten LD-bars on MCCs are labeled LD-1 through LD-10. Item 1 is the female SMAQ fiber connector with custom MCC heatsink. Item 2 and 3 are the laser diode anode and cathode, respectively. Item 4 is the BTS micro-optics. Items 5, 6, 7, 8, 9 are the slow axis cylindrical lenses, the turning mirrors, the turning prisms, the polarization beam combiner, and the aspheric lens, respectively. Image AA is the intensity of the focused beams at the fiber face. The red circle represents the 225µm diameter fiber core. Image BB is the image of the spatially combined collimated LD-bars. The red circle in image represents 0.22NA.

Download Full Size | PDF

Basically, the WL1000 consists of two sub-modules that are polarization beam combined using a beam-combiner as described by Faircloth [16]. Each sub-module is the mirror image of the other. The LD-bars are all S-polarized. The red lines indicate the optical beam from LD-3 and LD-8 for clarity. The polarization of LD-5 through LD-10 is rotated to P-polarization by a half wave plate.

Image AA is the intensity of the focused beams at the fiber face. The red circle represents the 225µm diameter fiber core. Image BB is the image of the spatially combined collimated LD-bars. The red circle in the image represents 0.22NA. Analysis of these images indicates ~10% of the light in the WL1000 exceeds the physical and numerical apertures of the fiber. This excess light is removed by a mode stripper as discussed below.

The performance of the WL1000 is shown in Fig. 3. The output power and PCE is plotted as a function of operating current. At an operating current of 130A, the coupled power is 1000W and the PCE is 50%.

 

Fig. 3 A) The output power from the WL1000 and the efficiency as a function of operating current. At 1000W from a 225µm, 0.22NA fiber, the efficiency is 50%. B) Spectrum of the WL1000 module at 140A. The FWHM is ~4nm.

Download Full Size | PDF

For Yb-doped fiber-laser pumping applications, only the power absorbed by the ytterbium is useful. The absorption per unit length of the fiber is determined by the dopant concentration and the fiber geometry. For high power applications, the active fiber length is shortened to limit degradation of laser performance by stimulated Brillouin scattering (SBS). Consequently, there is a trade-off between fiber length and absorbed pump power.

At 976nm, the Yb-absorption spectrum is ~1-2nm wide depending on the dopant concentration and the composition of the host glass. Figure 3(b) shows the spectrum of the WL1000 at 140A with a coolant inlet temperature of 25°C. The spectral center and the FWHM is ~977nm and ~4nm, respectively. A FWHM spectral width of ~4nm is sufficiently narrow for more than 95% of the pump light to be absorbed in Yb-doped 400µm double-clad fiber used in high power fiber amplifiers. The value of 95% is chosen to produce an optical-to-optical (amplifier output to pump power) efficiency in excess of 85%. The spectral center, which decreases ~0.3nm for each decrease of one degree in the temperature of the laser diode bars, can be shifted to 976nm by reducing the coolant inlet temperature to ~22°C. The narrow spectral width of the WL1000 is due to the uniform temperature across the individual MCCs, the tight tolerances of the LD-bars, the repeatability of the die-attach and the parallel flow of the coolants through the 10 MCCs.

The coupling efficiency, ~76%, is the ratio of the coupled power, 1000W, and the total power from the 10 LD-bars, ~1300W. The module efficiency, 50%, is the product of the efficiency of the LD-bars at the operating current of 130A, ~66%, and the coupling efficiency, ~76%. This coupling efficiency includes optical loss at the polarization beam combiner since the output of the LD-bars is not completely polarized. The module efficiency reaches a peak value of ~52% in the current range of 80-110A. Above 110A, there is a slow decrease in efficiency due to the reduction in LD efficiency as shown in Fig. 1 and a reduction in coupling efficiency from the well-known increase in slow axis divergence [17].

3. Connector

All high-power industrial lasers have detachable delivery fibers to eliminate the requirement of fusion splicing by the FCM user and the reliability issues associated with the splice. A SMAQ fiber connector is used in the WL1000 since it is the only commercially available, low-SWaP connector [18]. This connector has two highly-desirable attributes. The entrance to the connector is an AR-coated quartz block that significantly reduces the optical density at the entrance to the delivery fiber. Secondly, there is an integral mode-stripper to remove undesirable light that would otherwise be coupled to the cladding layer of the delivery fiber. This mode stripper removes more than 99% of the cladding-mode power.

The delivery fiber from a high power FCM must eventually be fusion spliced to a receiving fiber. The core diameter and numerical aperture of an optical fiber have tolerances. The fiber used in this study has diameter of 225µm ± 2µm and an NA of 0.215 ± 0.005 as specified by the manufacturer [18]. Even from the same spool, it is not possible to exactly match two ends of the fibers that need to be connected by fusion splicing. Depending on the mismatch, both light from the core and the cladding of the source fiber (cladding modes) are scattered at the splice [19, 20]. This scattered light must be absorbed and removed with a heatsink to prevent failure at the splice.

At 1000W from the fiber, the optical power located outside of the fiber core or numerical aperture, is estimated to be ~130W. At 1000W of cladding-mode free coupled-power, there is a total power of ~1300W from the 10 LD-bars. Approximately 5% of the light from the LD-bars is not captured or scattered by the BTS optics and another ~8% is not captured or scattered by the turning mirrors and polarization beam combiner. The heat from this “uncoupled” 130W of power, absorbed in various parts of the SMAQ connector, must be removed due to the use of epoxies in the construction of the connector. The glass transition temperature of epoxies is ~100°C. If the epoxy softens, the consequent motion of the fiber results in a thermal run-away at the connector [20, 21]. To prevent thermal damage to the SMAQ connector, a custom micro-channel heatsink was installed in the female portion of the connector to remove the heat from the absorbed power of the cladding modes and keep the temperature of the connector at a safe operating temperature, e.g. 50°C. From the ~20°C temperature rise of the SMAQ connector at 1000W from the fiber, the thermal impedance of the custom heatsink is estimated to be ~0.15°C/W.

4. Summary

The WL1000, the first 1000W, 50% efficient, 976nm fiber-coupled module, enables low-SWaP, 5-6kW fiber-amplifiers that can be coherently or wavelength beam-combined. The 225µm, 0.22NA delivery fiber is detachable using a commercially-available SMAQ connector to simplify fiber-amplifier assembly and to improve reliability by removing optical power in the cladding of the delivery fiber. The details of the WL1000 are: