1 Introduction

The major bottlenecks for increasing peak power in fiber lasers and amplifiers are material damage and deleterious nonlinear effects. This is due to very high-power densities within the small core size of the fiber. Recent development of large core air-clad photonic crystal fibers (PCFs) led to a significant increase of the damage and nonlinear effects thresholds, and currently, these fibers are considered as one of the most attractive alternatives in the field of high-peak-power fiber laser sources [1–6]. Nevertheless, systems based on PCFs suffer from several disadvantages compared to those based on conventional step-index fibers. The main disadvantage is incompatibility with “all-fiber” components such as Bragg mirrors, pump couplers, etc. This incompatibility originates from the very special structure of the PCF, including airholes in the inner clad and an outer air-clad. Accordingly, PCF-based lasers and amplifiers typically have a free-space design.

A common PCF laser configuration exploits free-space pumping through a dichroic mirror (DM). The main weakness of such pumping scheme is a common free-space optical axis of the pump and laser beams. This colinearity poses high risk for intense back reflections to the pump diodes and requires design compromises for each channel. Consequently, there have been many attempts to separate the laser and pump beam directions with various off-axis (sidewise) pumping schemes [7–11]. In the off-axis pumping approach, the pump and laser beams are separated and do not share the same optical axis (outside the fiber) before they are coupled into the fiber. However, most of the proposed methods suffer from low damage threshold due to adhesives and mechanical instability and often require special treatment and processing of the active fiber [10, 11].

In view of that, an off-axis pumping method, which exploits the extremely high numerical aperture (NA) of the PCF’s air-clad pump waveguide [12] in order to inject the pump beam in a large off-axis angle so that the laser and pump beams are separated, were recently proposed [13, 14]. This method dramatically decreases possible feedback from the laser to the pump diodes, has high damage threshold, and does not require any dichroic coatings, and special treatment or processing of the fiber. Furthermore, it allows scaling the number of independent pump sources, which leads to modularity and flexibility in design. Previously, we experimentally investigated this method only at low powers and did not fully characterize the performance.

In the present work, we experimentally demonstrate high-power free-space off-axis pumping of an active double-clad PCF, based on the method that was recently proposed by us [13]. We characterize the pump coupling dependence on the angle of incidence (AOI) and measure the laser performance and beam quality as a function of the AOI in a high-power CW oscillator configuration. Finally, we characterize the lasing feedback to the pump diodes.

2 Experimental setup

Our experimental PCF laser setup is schematically shown in Fig. 1. It is composed of a 2 m long, commercially available, Yb-doped double-clad polarization maintaining (PM) flexible PCF, with 40 μm core and 200 μm pump-clad diameters (NKT Photonics DC-200/40-PZ-Yb), which is coiled with the diameter of 40 cm; a 60-W fiber-coupled, narrowband (±0.2 nm), pump module operating at a center wavelength of 976 nm with 105 μm core and 0.15 NA delivery fiber; suitable high quality lenses to collimate the pump light and to couple it into the PCF; a DM to divert the laser output beam, which has high transmission at 976 nm and high reflection (HR) between 1,030 and 1,080 nm; a HR broadband flat rear mirror, which is positioned close to the 0 degrees cleaved bare fiber end; and the second bare fiber end is cleaved at 0 degrees and serves as an output coupler (OC) with ∼4% reflectivity. The focal lengths of the pump beam collimating lens (L1 in Fig. 1) and of the pump beam focusing lens (L2 in Fig. 1) are 40 and 50 mm, respectively. This results in a spot size (at normal incidence) of about 130 μm at the focus, allowing for efficient coupling of the pump beam into the fiber. Moreover, the output coupling side of the fiber (pump coupling side) is fixed on a rotary mount. Such arrangement enables to change the angle between the optical axis of the PCF and optical axis of the pump beam (defined here as AOI), as shown in Fig. 1. An AOI of 0 degrees denotes the on-axis case, and other AOIs represent off-axis cases. It should be noted that DM is mandatory in on-axis experiments, but it was used also in the other experiments in order to accurately compare the results in the different cases. Finally, such an arrangement enables efficient double pump-pass, which drastically increases the pump absorption (the actual propagation length of the pump beam in the fiber is about 4 m).

Fig. 1
figure 1

Schematic experimental setup for off-axis pumping of a PCF laser

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3 Experimental procedure and results

First, in order to measure the pump coupling efficiency as a function of AOI, we removed the HR mirror and replaced the active Yb-doped PCF with a similar commercially available un-doped passive PCF (NKT Photonics DC-200/40-PZ-Si). The measured coupling efficiency as a function of AOI, normalized with respect to the Fresnel reflections from both facets of the 0 degree cleaved bare fiber for un-polarized pump beam, is shown in Fig. 2. The squares represent AOI variations along the fast-axis polarization plane of the fiber, and triangles along the slow-axis plane. Here, slow-axis polarization plane is defined as the plane that contains the stress rods, and fast-axis polarization plane is perpendicular to it.

Fig. 2
figure 2

Coupling efficiency as a function of AOI in a corresponding passive PCF

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As evident from Fig. 2, there is a negligible degradation in coupling efficiency with increasing AOI up to 20 degrees. At higher AOIs, the coupling efficiency clearly depends on the input plane. When the pump light is initially launched in the fast-axis polarization plane, the coupling efficiency is still very high, and >97 %. But when the pump light is launched in the slow-axis polarization plane, the coupling efficiency is degraded at large angles. This may be explained by the different overlap of the pump beam with the stress rods for both input planes. Nevertheless, even with the slow-axis polarization plane, the coupling efficiency at relatively large AOIs is very reasonable (>95%). Accordingly, the coupling efficiency with the proposed off-axis pump coupling method is very effective and does not suffer from significant losses. It is important to note that most high-peak power fiber lasers and amplifiers work with endcapped fibers in order to avoid damage to the fiber end facet. In these cases, if the off-axis method is employed, the endcaps should be properly designed to allow the pump beam a "clear path" [13].

The measured performance of the PCF laser for several AOIs is shown in Fig. 3. The examined AOIs are 0, 6, 22, and 28 degrees, and they are presented as squares, dots, triangles, and flipped triangles, respectively. Furthermore, the calculated slope efficiencies are listed in parentheses in the legend. It can clearly be seen that for AOIs below 30 degrees, the AOI has no significant effect on the laser performance. Moreover, a high output power of ∼46 W with a slope efficiency of ∼80 % was achieved for all examined AOIs. The differences in lasing thresholds and slope efficiencies are dismissive. These results verify that with high pump powers and off-axis angles as large as 30 degrees, there is no degradation in laser efficiency compared to standard on-axis pumping.

Fig. 3
figure 3

Measured laser output power as a function of launched pump power for several AOIs

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In our previous work, we mentioned that off-axis pumping may result in increased pump absorption [13]. However, in the current study, we did not observe any effect of the off-axis pumping on the pump absorption. This may be explained by the fact that in the present study, we are using relatively long active fiber with a core diameter of 40 μm, in contrast to the previous study where we used a short active medium (55 cm long) with a core diameter of 70 μm

Next, we measured the laser output beam quality parameter (M 2) by means of the 4σ method at high-power output (46 W). Figures 4a and b show the measured near- and far-field intensity distributions, respectively, for the on-axis case (AOI=0). Figures 4c and d show the near- and far-field intensity distributions, respectively, for the off-axis case with AOI=28 degrees. The beam quality, which is calculated from these distributions, is 1.1 and 1.07 for on-axis and off-axis cases, respectively. Furthermore, we performed a beam profiling measurements, which are shown in Fig. 4e. Following these measurements, the beam quality is 1.07 and 1.03 for on-axis and off-axis cases, respectively. These results indicate that the beam quality is not affected by the pumping configuration and is TEM 00.

Fig. 4
figure 4

Measured near- and far-field intensity distributions of the output laser beam. a and b near- and far-field, respectively, with the on-axis configuration; c and d near- and far-field, respectively, with the off-axis configuration. e Beam profiling measurements: beam radius as a function of the distance from the waist. The blue and green dotted curves represent M 2 dependent Gaussian fit for the on-axis (circles) and the off-axis (squares) cases, respectively

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One of the major issues in fiber lasers is to protect pump diodes from the harmful lasing feedback penetration. With respect to this point, we characterized the amount of feedback from the laser to the pump diodes. In order to do that, we scanned a fiber tip across the output beam to collect the output signal at various angles, when the pump AOI is 28 degrees. Then, the signal from this probe was send through a fiber to an optical spectrum analyzer. The scanning includes 26 points at different angles relative to the center on lasing optical axis. Each point has wavelength dependent data, which were integrated over the wavelength in the lasing range (from 1 to 1.1 μm), and the result of this integration is shown in Fig. 5.

Fig. 5
figure 5

Characterization of the lasing content as a function of the relative angle to the optical axis of the lasing. The off-axis region represents the acceptable angles of the pump focusing lens L2, and the core NA region represents the core mode corresponding to angles

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First, it can clearly be seen that the difference between core mode and clad mode intensity densities is −80 dB. Second, taking into account the dimension and the position of optical elements (in particular pump focusing lens L2, which is responsible for collecting of the lasing back to the pump diodes), and the free-space beam propagation optics, we found that our off-axis pump method inserts losses to the back-reflected laser light equal to ∼54 dB. This means that there is lasing light in the fiber clad that can reach the diodes, but it is very low compared to the on-axis lasing. Thus, our off-axis pumping method is equal to approximately three DMs compared to the on-axis conventional method in term of isolation between lasing and pump diodes. Additionally, since each DM inserts losses also to the pump beam (∼0.27 dB in our case), with our pumping method, higher amount of pump energy reaches the active fiber.

Finally, the spectral content of the output with different gain conditions is measured. The results of these measurements are shown in Fig. 6. Since our cavity does not include any spectral selective elements, we observed unstable behavior of the spectrum; thus, each measurement in Fig. 6 is actually an average of 10 measurements. It can clearly be seen that the higher the gain the lower the dominant lasing wavelength of the laser. Finally, we observed that the spectral content is independent of the pumping configuration (off-axis or on-axis).

Fig. 6
figure 6

The spectrum content of the laser output at different output powers: solid (black) curve for 8 W, dotted (green) curve for 22 W, and dash dotted (blue) curve for 45 W

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4 Discussion and conclusions

Our results show that at high power and at relatively large pump AOIs, the performance of the PCF laser is not degraded. In terms of the lasing threshold and slope efficiency, it is as good as the conventional on-axis pumping method. In terms of the pump coupling, it enables very high pump coupling efficiency, which is higher than that offered by most of the all-fiber non-PCF pump couplers. Furthermore, we observed a slight decrease in pump coupling efficiency at AOIs larger than 20 degrees, which we believe corresponds to the position of the Boron-doped stress rods with respect to the pump direction. Moreover, the output beam quality is not affected by the AOI and is single mode.

It is important to note here that with the off-axis pumping design, there is a limitation on the amount of pump power that can be delivered to the fiber. This limitation is due to the partial filling of the pump NA and can be encountered at very high pump powers. However, already today, one can find commercial fiber-coupled pump modules with an output power of 100 W, in a 100 μm core and 0.15 NA. Using several such modules, from different directions in an off-axis pumping configuration, it may be possible to deliver ∼600 W to a single fiber facet. Accordingly, our off-axis pumping scheme is suitable for applications that need moderate powers that are <1.2 kW of pumping, but is not suitable for ultra-high-power applications.

Finally, in the case of pulsed lasers, the pump diodes protection is a significant issue and it seems that our results are valid also for most high-peak-power lasers. We believe this to be the case because our measurements are based on the angular relative distribution, which is independent on specific work conditions such as the output peak power.

To summarize, to the best of our knowledge, we report for the first time an experimental demonstration of off-axis pumping of a PCF laser at relatively high CW powers. We believe exploiting the proposed off-axis pumping method in high-peak-power PCF lasers will result not only in robust operation with high-level protection to the pump diodes, but also in higher wall-plug efficiency.