1. Introduction

High-power lasers with high beam quality are required in many applications, such as basic scientific research and precise machining. However, a single high power laser often suffers from poor beam quality, instability, and heat dissipation. The power and beam quality competes with each other. Coherent combination [1–3], namely addition, of the output beams of several low power lasers into a single beam provides a possible way to resolve the problem, which permits to extend the use of an available technology and of an already optimized laser. Coherent combination is particularly attractive for compact, rugged, high efficiency fiber-based lasers that have output powers limited by the small transverse dimensions of the gain mediums. Fiber-based lasers have recently attracted considerable attention [4–7] as a high-power solid-state laser system that partially alleviates the damage of thermal effects in high power bulk lasers. Yet, the output power from a single fiber laser is ultimately limited by the onset of nonlinear effects such as stimulated Raman scattering, stimulated Brillouin scattering, and fourwave mixing. Further increase in output power will require coherent combination of the outputs from multiple fiber lasers. There are efforts underway to combine multiple elements into a compact, spatially coherent source [8–14].

Although the techniques of combining the output power of multiple fibers into a single fiber through fiber couplers do not remove the limitation on the ultimate power of a single fiber, coherent combination of all-fiber lasers have still attracted much attention [15–21], owing to the proved advantages of all-fiber lasers in compactness, efficiency, reliability and beam quality. Michelson [15–18] and Mach-Zehnder [16,19,20] type resonators have been successfully utilized to reach high combining efficiency with up to eight fiber lasers [21]. In spite of the constant and random changes in the optical path length of the individual elements and the interferometric architecture, the lasers operate stably, thanks to the self-adjust ability provided that the optical path length difference is enough [19]. The resonant frequencies of the circulating fields in these laser cavities self-adjust so that the intensity in the common arm is the maximum. This all-fiber combining technique has also been demonstrated to be compatible with widely tunable [19] and Q-switched [20] fiber lasers, by adding a diffraction grating or an acoustooptic modulator at the common arm of the interferometer. Generally speaking, the utilization of the diffraction grating for wavelength tuning destroys the all-fiber property of the laser.

In this paper, the tunable function of a Mach-Zehnder type fiber laser is realized by inserting a fiber pigtailed tunable bandpass filter (TBF) into one branch arm instead of into the common arm as usual. This amendment improves the efficiency of the tunable laser without evident spectral quality penalty, mainly thanks to the fact that only one of the arms suffers from the insertion loss caused by the TBF. The technique is estimated to have applications in the tuning of an extremely high power laser with a low power tunable filter, and probably can be extended to some other applications such as coherent combining of linearly polarized fiber lasers and bulk lasers.

2. Experiments

The experimental setup has been schematically shown in Fig. 1. The TBF is inserted into the Mach-Zehnder type fiber laser at one of the four points indicated by the dashed frames. An ordinary arrangement inserts the TBF into the common arm of the resonator, i.e. point I and II. In this paper, the TBF is inserted into one branch arm of the laser, i.e. point III and IV. The distinction between I (III) and II (IV) is that the TBF is either near the output port or the high reflectivity (HR) port as compared with the EDF. We call the laser as an output port common arm, HR port common arm, output port branch arm, and HR port branch arm filtered laser respectively for the TBF at point I, II, III, and IV. As has been demonstrated in previous works [19], the optical path length difference of the two arms needs not to be precisely controlled. In this work, the fiber length difference of the two arms is about 1m without precise control. The total fiber length of each arm is around 20m. The TBF is a commercial standard single mode fiber pigtailed one, with insertion loss of ~2.2 dB, -3 dB bandwidth of ~0.5 nm and -20 dB bandwidth of ~2.0 nm. The rated tuning range of the TBF is from 1540 nm to 1570 nm.

 

Fig. 1. Experimental setup of the tunable Mach-Zehnder type fiber laser. The TBF is arranged at one of the four points indicated by the dashed frames. TBF: Tunable Bandpass Filter; PC: Polarization Controller; LD: 110 mW 980 nm Laser Diode; WDM: Wavelength Division Multiplexer; EDF: 7.2m Erbium Doped Fiber; C: 3 dB Coupler; HR: High Reflectivity

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Fig. 2. Output power and spectra properties of the tunable fiber lasers. η_s: slope efficiency.

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The power properties of the lasers are shown in Fig. 2(a). The branch arm filtered laser is obviously efficient than the common arm filtered laser for both the HR port filtering and the output port filtering (compare the stars with the triangles, or the dots with the rectangles). The slope efficiency of the common arm filtered laser is almost equal to that of a corresponding individual component tunable laser, indicating a high combining efficiency. However, the branch arm filtered laser is more efficient than a corresponding individual component tunable laser. The maximum output power of the branch arm filtered laser is more than twice that of the corresponding individual component tunable laser. The improvement mainly thanks to the fact that only one of the two arms suffers from the insertion loss of the TBF in the branch arm filtering configurations. Note that in the common arm filtering configuration, both arms suffer from the insertion loss. The enhancement of the slope efficiency is 3% and 8%, respectively, for the HR port and the output port filtering. The maximum output power increases 11% and 35%, respectively, for the HR port and the output port filtering. These values are actually depended on the loss of the TBF. The technique is especially effective for TBF of large losses. If the branch arm filtering technique is applied to a multiple-arm coherent combination, the improvement of the slope efficiency will be even larger, because only one of the multiple arms suffers from the insertion loss of the TBF, yet all of the arms suffer from the insertion loss in the conventional common arm filtering configuration. The scheme of scaling is discussed in the conclusion section.

Figure 2(b) shows the laser spectra of the various kinds of lasers at maximum pump power. The laser bandwidth at -3 dB is 0.09 nm for all of the four configurations. The only distinguish between the spectra is the slightly changed side mode suppression ratio (SMSR). Although the TBF only filters lights that pass through the lower arm of the interferometer in the case of the branch arm filtering, the laser oscillates at the pass band of the TBF stably with a large SMSR, due to the self-organization property of lasers ensuring operation on modes of lowest losses [22]. The coherent strengthened wavelengths within the pass band of the TBF almost suffer no loss when pass through the 3 dB couplers, while the wavelengths beyond the pass band suffers a loss of 3 dB every time they pass through the couplers. Furthermore, the former is amplified via both arms, while the latter is only amplified via the arm without the filter.

It should be noted that the two amplifiers are equally pumped in all of the four configurations. Although the two arms suffer from different losses that results in a power imbalance in the branch arm filtered lasers, the combining efficiency is still high due to the insensitivity of the combining efficiency to the power imbalance [17,19]. In a word, the branch arm filtering technique improves the efficiency of the laser without obvious penalty in spectral quality. In the following, a detailed investigation on the HR port branch arm filtered laser will be reported, which is the most efficient one.

3. Detailed investigation of the HR port branch arm filtered laser

The laser spectra of the HR port branch arm filtered laser are affected deeply by the pump power of the tuning arm (P_LD2), yet changes a little with the pump power of the free running arm (P_LD1). Figure 3 shows the spectra variation with P_LD2, at the maximum pump power of P_LD1. Laser emerges around 1532 nm and 1557 nm when P_LD2=0. When P_LD2 increases, laser also emerges from the pass band of the TBF that has been randomly set to 1549.13 nm. Then, the laser peaks around 1532 nm and 1557 nm are thoroughly suppressed when P_LD2 becomes larger than 5.2 mW. Further increase of P_LD2 leads to the increase of the SMSR till the maximum value. Note that there is a special threshold value of P_LD2. Undesired noisy laser oscillation appears below this value and disappears above this value. We call this value of P_LD2 as the suppression power in the rest of the paper, which is an important value in the branch arm filtered laser. The coherence of lights from the two arms only occurs when P_LD2 is larger than the suppression power. P_LD2 must works above the suppression power in order to obtain a stable laser without noisy laser oscillation.

 

Fig. 3. Spectra of the HR port branch arm filtered laser under P_LD1=110mW. The purple, green, blue and red lines correspond to P_LD2=0, 3.8, 5.2, and 110 mW, respectively.

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The suppression power changes with wavelengths as shown in Fig 4(a). The suppression power is only 0 mW around 1560 nm, due to the high gain and low absorption loss of the EDF at these wavelengths. The suppression power increases rapidly at the tuning edge of the laser because of the relatively lower gain at these wavelengths. The SMSR indicated by the blue dots in Fig. 4(a) also changes with wavelength. Yet the trend of the SMSR and the suppression power is reversed. The SMSR reaches the maximum near 1560 nm, while the suppression power reaches the minimum near 1560 nm. Fig 4(b) shows the suppression power versus P_LD1, at the wavelengths of 1530, 1550, and 1570 nm. At 1550 nm, the suppression power is only 5.2 mW, and is almost unchanged with increasing P_LD1. As for 1530 nm and 1570 nm, the suppression power changes almost linearly with P_LD1. The suppression power reaches the maximum at 1570 nm in the tuning range of 1530 nm to 1570 nm, according to Fig. 4(a). The slope of the green line in Fig. 4(b) is 46%. By combining the above two conditions, it is concluded that the ratio of P_LD2 to P_LD1 should be larger than 0.46, in order to obtain laser spectrum without noisy laser oscillation over the whole tuning range. When the two arms are equally pumped as usual, that is P_LD2/P_LD1=1, the suppression condition is over satisfied. The laser is very stable at P_LD2/P_LD1=1 over the whole tuning range under ordinary laboratory conditions, as shown in Fig. 5. That is, particular notice to the suppression power is unnecessary in practical applications.

 

Fig. 4. Suppression power versus wavelength and P_LD1. Rectangles in (a) are measured with P_LD1 at its maximum power. Blue dots in (a) are the SMSR of the laser with both LDs at their maximum power. The suppression power means the minimum P_LD2 needed to thoroughly suppress undesired noisy laser oscillation.

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Fig. 5 Continuously scanned spectra of the HR port branch arm filtered fiber laser.

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Fig. 6. Power and spectra versus wavelength of the HR port branch arm filtered fiber laser. (b) and (c) correspond to the PC optimized at 1550 nm and at each wavelength, respectively.

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The data in the preceding are obtained under the condition of optimizing the output power by adjusting the polarization controller (PC) at each wavelength. The suppression power increases and the SMSR decreases if the PC is not optimized. It is necessary to optimize the PC at each wavelength to obtain a uniform output power and fine shaped laser spectrum during the wavelength tuning process. Fig. 6(a) shows the output power versus wavelength. The red rectangles are measured by optimizing the PC at each wavelength. The blue dots are measured by only optimizing the PC at 1550 nm. In the latter condition, the output power degrades to almost half of the maximum value at some wavelengths. However, the power changes a little if the PC is optimized at each wavelength. The power varies within 2.4dB and 0.3dB, respectively, for the PC optimized at 1550 nm and at each wavelength. The data are a bit similar to those reported in [19], note that the y-axis is logarithmically scaled in that paper. Figure 6(b) and 6(c) shows a few typical laser spectra during the tuning process. As Fig. 6(b) shows, if the PC is not optimized at each wavelength, undesired laser oscillation occurs at the long wavelength tuning edge of the filter, and the SMSR slightly decreases at most wavelengths. The laser can then only be tuned from 1530 nm to 1569.5 nm without undesired noisy laser oscillation. The tuning range decreases about 0.5 nm. However, if the output power is not strictly required over the tuning range in practical application, it is okay to only optimize the PC at 1550 nm for convenience.

4. Conclusion

In conclusion, the TBF is inserted into one branch arm of the Mach-Zehnder type resonator instead of into the common arm as usual, in the coherent combining of two tunable fiber lasers. The slope efficiency is improved because only one arm suffers from the insertion loss of the TBF. The laser can be tuned from 1530 nm to 1570 nm, which is restricted by the tuning range of the TBF. The output power changes within 2.4 dB during the tuning process if the PC is only optimized at one wavelength, while changes within 0.3 dB if the PC is optimized at each wavelength. The suppression power, i.e. the minimum pump power of the tuning arm needed to thoroughly suppress undesired noisy laser oscillation, changes with wavelength and the pump power of the free running arm. When the two arms are equally pumped, particular notice to the suppression power is unnecessary. The SMSR of the output laser remains above 52 dB over the whole tuning range. The laser is stable in power and spectrum under ordinary laboratory conditions. The technique has the advantage of tuning high power laser with a relatively low power filter.

The branch arm filtering technique is estimated to be scalable to the combination of a large number of lasers with even bigger improvements in efficiency. We propose two scaling schemes here, as shown in Fig. 7. Figure 7(a) is an ordinary scheme. With this scheme, the number of lasers can be increased to 4, 8, 16…. Thus the TBF can be utilized to filter laser with output power much larger than that it can bear. The scaling scheme in Fig. 7(b) is firstly proposed to the best of our knowledge. A Mach-Zehnder type fiber laser is treated as one branch of another Mach-Zehnder interferometer, while an individual fiber laser with double power is treated as the other arm. The whole laser is then combined with another four times powered individual laser by two 3dB couplers. In such a way, the laser can be scaled to arbitrary number of branch arms with output power increases exponentially proportionally to the number of the branches. Such arrangement needs a series of laser modules with exponentially increased power. The obvious advantage is that with this configuration we can easily control the wavelength of an extremely high power laser with a low power TBF. In both configurations shown in Fig. 7, the slope efficiency will be improved as compared with the common arm filtering technique. Furthermore, the increased values are estimated to be larger than those reported here, because only one of the multiple arms suffers from the insertion loss of the TBF, yet all of the arms suffer from the insertion loss in the conventional common arm filtering configuration. However, the number of total scalable arms is likely to be limited by the needed suppression power, which should be proved through further experiments.

 

Fig. 7. Scaling schemes of the branch arm filtering technique.

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Although not yet proved by experiments, the branch arm filtering technique including the proposed two scaling schemes are expected to be compatible with coherent combining in Michelson configuration, and probably can be extended to some other applications such as coherent combination of linearly polarized fiber lasers and bulk lasers as well.