1. Introduction

Single-frequency lasers at 1.5 μm are essential tools for a variety of applications in spectroscopy, optical communications, and optical sensing. In many cases, such as free space optical communications [1], laser radar [2], satellite based remote sensing [3,4], distribute fiber-optic sensors [5], and frequency domain reflectometry [6], the requirements may become extremely demanding in terms of power, compactness, insensitivity to environmental disturbance, and high temporal coherence. Bulk cavities exploiting large modal volumes are able to provide high output powers, but generally are quite sensitive to technical noise and are not compact devices. Semiconductor lasers provide monolithic and compact single-frequency sources, but the relatively wide linewidth sets a limit to the temporal coherence achievable. Short fiber lasers [7,8] and waveguide lasers [9,10] are compact devices with monolithic structures which have the potentialities to satisfy all the requirements previously outlined.

In this paper we report on a waveguide laser in a very compact linear cavity configuration able to emit more than 50-mW output power in single longitudinal and transverse mode operation. The waveguide was written in an Er:Yb-heavily-doped phosphate glass by means of ultra-short laser pulses, which is an emerging technology allowing for fast prototyping, flexible piece-by-piece and 3D fabrication capabilities [11,12].

2. The waveguide laser set-up

2.1 The fs-written waveguide active medium

Figure 1 shows a schematic of the laser cavity. The active medium is a 20-mm-long channel waveguide fabricated by femtosecond laser writing in a phosphate glass substrate doped with 2% wt of Er₂O₃ and 4% wt of Yb₂O₃. Dopant concentrations and sample length have been optimized to obtain high gain per unit length in order to fabricate very compact and efficient devices. The writing system is based on a diode-pumped, cavity-dumped Yb:KYW oscillator at 1040 nm [13]: a transverse writing configuration, in which the sample is translated by motorized stages in a direction perpendicular to the laser beam, has been adopted. A high numerical aperture objective (100x with oil-immersion) is used to focus the pulsed laser inside the glass substrate. Following the results presented by the authors in a recent paper [14], an optimized set of writing parameters was adopted to fabricate the waveguide, namely, 505 kHz repetition rate, 436 nJ energy per pulse and 100 μm/s writing speed.

 

Fig. 1. Waveguide linear laser cavity. A bi-directional pumping scheme is adopted. Pump interaction is removed by means of an optical isolator (ISO) and a half-wave polarization controller (PC λ/2).

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The waveguide end-facets were polished after laser inscription and the waveguide was then fully characterized at 1590 nm (outside the erbium absorption band) in terms of passive guiding properties. In particular, by using an IR camera, the near field intensity profile of the guided mode is acquired, providing a 1/e² diameter of about 11.1 μm. From the overlap integral between the experimental intensity profiles of the waveguide and fiber modes, the theoretical coupling loss to standard telecom fibers was estimated to be CL = 0.1 dB/facet. Following the method reported in [14] we also measured the waveguide insertion loss IL = 1 dB, thus propagation loss was estimated to be at worst PL = (IL -2CL)/L = 0.4 dB/cm.

A characterization of the active properties of this waveguide has been also performed using the set-up described in [14]. For a bi-directional pumping configuration with 510 mW of total pump power, a peak net gain of 5.7 dB at 1535 nm has been measured.

2.2 The waveguide laser cavity configuration

The waveguide laser cavity employs a linear configuration where the active waveguide is butt-coupled on both sides to fiber Bragg gratings (FBGs) fabricated in standard single-mode-telecom fibers. A broad-band flat top FBG with 1-nm bandwidth (FWHM) provides high reflectivity (99.8%) at one side, and a narrow-bandwidth FBG with about 0.1-nm FWHM is used as output coupler. An index-matching fluid able to support high power density at 980 nm is inserted between waveguide and fiber ends. Two fiber pigtailed InGaAs laser diodes with a bi-propagating pumping scheme supply up to 510 mW incident pump power (260 mW at 980 nm from pump 1 and 250 mW at 976 nm provided by pump 2) through 980/1550 nm wavelength-division-multiplexers (WDMs). In order to remove the parasitic interaction between the counter-propagating pumping beams, a single-stage optical isolator is connected to pump 1, and, in addition, a half-wave fiber polarization controller is inserted along the fiber connecting pump 2 to the waveguide. By rotating the axis of the polarization controller, the polarization of the counter propagating pumping beams is set orthogonal to each other, thus avoiding any interference which is detrimental for the pump efficiency.

3. Experimental results and discussion

3.1 Optimum output coupling investigation

In order to experimentally investigate the optimum output coupling of the waveguide laser we performed a set of measurements of the laser input-output characteristic with several output couplers using FBGs with reflectivities ranging from 20% to 90%.

 

Fig. 2. Measured slope efficiency (triangles) and pump power threshold (circles) as a function of cavity output coupling.

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The FBGs have the same bandwidth (∼ 0.1 nm corresponding to 12.5 GHz at FWHM) and comparable fiber-pigtail length, keeping the overall cavity length L almost constant to a value of about 0.5 m. Under this conditions, the resulting free spectral range (FSR) of the cavity, ∼200 MHz, is too low to achieve single mode operation and in fact the laser was observed to operate in multi-mode regime with an oscillating bandwidth of about 2 GHz. Figure 2 shows the measured slope efficiency values (triangles) and pump power thresholds (circles) as a function of the output coupling. The maximum slope efficiency of 21.5% was obtained with a corresponding threshold pump power of 142 mW employing a 61% output coupler. In this conditions, a maximum output power of 80 mW was obtained under 510 mW incident pump power, thus outperforming previous results obtained with a similar though not completely optimized laser cavity [15].

3.2 Single longitudinal mode operation

In order to achieve single mode operation, the output coupler was replaced by a narrow bandwidth fiber Bragg grating and the cavity length was reduced by cutting the FBG fiber. Among the available fiber Bragg gratings, we selected the one providing the narrowest bandwidth and an output coupling as close as possible to the measured optimum value. Our choice was a 57% output coupler with 64 pm FWHM bandwidth (corresponding to about 8 GHz). After cutting the FBG fiber, a linear laser cavity of 5.5 cm length was obtained. Such a compact cavity results in a large FSR of 1.82 GHz thus consistently reducing the number of modes falling within the FWHM bandwidth of the FBG.

 

Fig. 3. Input-output characteristic of the 5.5-cm long waveguide laser cavity. Single mode operation provides up to 55 mW output power (circles). A maximum output power of about 80 mW is achieved in a slightly multimodal regime (triangles). Inset shows the relative intensity noise (RIN) of the laser when operating in single mode at 50 mW output power.

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In this condition, single frequency operation was easily obtained by making one longitudinal mode coincident with the reflectivity peak of the output FBG, being all the adjacent modes suppressed by the frequency depending losses of the narrow bandwidth output coupler.

Figure 3 shows the input-output characteristic of the 5.5-cm long laser cavity. The threshold pump power was 124 mW with 21% slope efficiency. To ascertain single mode operation we monitored the laser power spectrum by means of a scanning confocal Fabry-Perot interferometer (Burleigh, mod. SA-800). The interferometer has a fixed free spectral range of 8 GHz, a finesse of 100, and the signal to noise ratio on the photodiode was about 26 dB. Single mode operation was maintained till a maximum pump power level of about 400 mW, corresponding to an output power of about 55 mW (see Fig. 4.a). At higher pump power a slightly multimodal regime appears, with weak side peaks 1.8-GHz apart from the central mode, thus corresponding to adjacent longitudinal modes (see Fig. 4.b). The very low intensity of these modes is confirmed by the fact that no change in the laser slope efficiency between the single-mode and multi-mode regime was observed, as reported in Fig. 3.

The inset in Fig. 3 shows the relative intensity noise (RIN) trace of the laser operating in single longitudinal and transverse mode at 50 mW output power, recorded by means of a fast photodiode (Newfocus, 125 MHz bandwidth) connected to an electrical spectrum analyzer. The RIN was dominated by a peak of about -60 dB/Hz, located at 490 kHz, corresponding to the relaxation oscillation frequency of the cavity. As compared to previously reported waveguide lasers fabricated with the same technology [15], this cavity suffers from a higher intensity noise. Actually, the use of a narrow bandwidth FBG make the single-mode laser RIN quite sensitive to cavity length fluctuations induced either by mechanical vibrations at the fiber-waveguide butt-coupling interface and by environmental temperature variations. We expect that a significant improvement in terms of power stability could be achieved by using fiber-pigtailing technique to permanently connect the waveguide to FBGs, and by employing a Peltier cell to stabilize the length of the cavity against thermal variations.

 

Fig. 4. Fabry-Perot laser spectrum of the 5.5-cm long laser cavity operating (a) in single longitudinal mode at 55 mW output power, and (b) in slightly multimodal regime at 80 mW output power.

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4. Conclusions

In conclusion, we demonstrated a very compact and efficient waveguide laser at 1.5 μm, based on a femtosecond laser written waveguide, providing up to 55 mW maximum output power in single longitudinal and transverse mode. This source is a promising candidate for several laser-based systems where high output power, single frequency operation and reduced size are simultaneously required.