1. Introduction

Narrow-linewidth high-power lasers find application in fields like coherent optical communication, spectroscopy, precision measurements, or laser cooling. Today, optically pumped solid state lasers or fiber lasers are widely used for such applications. Unfortunately, these systems suffer from low efficiency, poor mechanical stability, large size, and heavy weight. Semiconductor lasers overcome these shortcomings. Furthermore, (frequency doubled) semiconductor lasers can cover most of the visible and NIR spectral range and offer direct high bandwidth modulation capability. However, the linewidth requirements of the above mentioned applications have not been met so far by diode lasers at the 1 Watt level.

Spontaneous emission into the lasing mode is the dominant contribution to the intrinsic spectral linewidth of most semiconductor lasers [1]. The linewidth caused by spontaneous emission events is inversely proportional to the number of photons in the laser mode and therefore decreases with the inverse of the optical output power (1/P-dependence). According to Henry [1], narrow-linewidth operation of semiconductor lasers can therefore be obtained by either increasing the cavity Q-factor or by increasing the optical output power. Additionally, contributions like spatial hole burning [2] and side mode partition noise [3] may also broaden the linewidth if the photon density is fluctuating within the cavity or if the side mode suppression ratio is not sufficiently high.

The spectral linewidth is further increased by technical noise (”technical linewidth”), which is assumed to be power independent [4, 5]. The most important sources of technical noise are injection current fluctuations, temperature noise, or mechanical vibrations.

Monolithic semiconductor lasers typically do not feature stable single mode, mode-hop free tunable, narrow-linewidth operation at output powers in the range of 1 W and beyond, since the side mode suppression ratio is usually not sufficiently high and the linewidth is broadened by spatial hole burning [6] and by increasing injection current noise (inherent to high-power diode laser drivers). Furthermore, single mode operation, which is essential for narrow-linewidth emission of solitary laser diodes requires a tight confinement of the optical wave in lateral as well as in vertical direction. At high optical power operation this leads to power densities within the laser cavity and at the facets that can cause catastrophic optical damage (COD) [7]. A monolithic master oscillator power amplifier (MOPA) concept with a tapered amplifier can provide single-mode operation at high output power [8,9], however, the spectral properties like emission frequency and linewidth of such systems are not stable due to optical feedback effects [10] and coupling of amplified spontaneous emission (ASE) from the amplifier section to the oscillator [11]. In a bench-top setup an optical isolator between the amplifier and the oscillator can suppress feedback effects [12, 13], however, this increases size and weight and degrades mechanical stability.

Here, we demonstrate a compact (footprint: 50 x 10 mm²), narrow-linewidth (100 kHz FWHM, 3.6 kHz intrinsic), high-power (1 W) master-oscillator power-amplifier laser module based on semiconductor components. The output of the oscillator, which is optimized for narrow-linewidth emission, is amplified without any significant modification of its spectral stability. Furthermore, we explain the heterodyne measurement technique used to analyze the linewidth of the laser sources.

2. Linewidth measurement technique

The contribution of spontaneous emission to the linewidth can be described by a white frequency noise spectrum and leads to a Lorentzian lineshape (the linewidth of which is considered the intrinsic linewidth). Often, the technical noise can be described by a 1/f frequency noise spectrum and then resembles a Gaussian lineshape to a good approximation [14, 15]. The FWHM (full width at half maximum) linewidth is determined by both, the intrinsic and the technical linewidth. The technical noise typically dominates the FWHM linewidth at high output power due to the 1/P-dependence of the intrinsic linewidth. Therefore, determining only the FWHM linewidth is not sufficient to characterize the spectral stability of the laser. Rather, the application one has in mind determines whether the FWHM linewidth or the intrinsic linewidth is relevant. For coherent optical communication the intrinsic linewidth is more important [16] whereas for applications like spectroscopy the FWHM linewidth is relevant.

A heterodyne linewidth measurement method is used for characterization of the linewidth [17, 18]. The emission of two independent MOPA modules is superimposed on a photodiode to obtain the beat note spectrum. To cancel out any drifts of the beat note deduced from the two free-running lasers, a “weak” frequency locking scheme with a bandwidth of 6 kHz is applied to lock the frequency of one module to that of the other at an offset of approximately 56 MHz. Commercially available low noise current sources (LDC-3724B, ILX Lightwave) together with low noise filters (LNF-320, ILX Lightwave) are used to drive the oscillators. Optical isolators with an isolation of 60 dB are used to suppress optical feedback from the measurement setup. Frequency noise measurements have been performed with a spectrum analyzer (FSV, Rohde & Schwarz). The acquisition of the frequency noise spectra is explained in more detail in [18]. The FWHM linewidth of the beat note is derived from the beat note spectrum itself. The measurement timescale of the FWHM linewidth is determined by the bandwidth of the “weak” lock and corresponds to 170 μs. The intrinsic linewidth of the beat note is obtained by multiplying the average value of the white noise floor of the frequency noise spectrum by a factor of 2π. Note that the beat note spectrum consists of a convolution of two spectra. Therefore, the intrinsic linewidth needs to be divided by a factor of 2 and the FWHM roughly by a factor of 2^0.5 (assuming that the FWHM linewidth is mainly determined by technical noise and hence corresponds to a Gaussian lineshape) to obtain the linewidths of a single laser.

3. Design and fabrication of the laser source

In this work we present a micro-integrated MOPA concept that overcomes the problems mentioned in the introduction and provides a diode laser system with narrow-linewidth, high-power emission, which is suitable for operation in a rugged environment, e.g. in space. The concept of the micro-optical bench is depicted in Fig. 1a. The output beam of a master oscillator (MO) with excellent spectral properties is collimated by a pair of aspherical cylindrical micro-lenses. The collimated beam passes a micro-isolator to suppress optical feedback, and is then coupled into an amplifier by a second pair of micro-lenses. The amplified light is collimated by a three lens system in order to obtain a round beam and thus ease fiber coupling. The light which is emitted from the rear facet of the oscillator is collimated and dumped to avoid feedback effects.

 

Fig. 1 (a) Micro-optical bench where all semiconductor components and the optics of the MOPA module are fitted to. The MOPA consists of a DBR laser as oscillator and a power amplifier which comprises of a ridge waveguide entry and a tapered amplifier section. A pair of aspheric cylindrical lenses is used to collimate the output of the MO and one more to focus into the entry section of the PA. A micro-isolator between the MO and the PA suppresses optical feedback. Three aspheric cylindrical lenses are used to obtain a round, collimated beam. (b) Packaged micro-integrated MOPA module with connectors and copper heat sink. The inset shows the oscillator and the micro lenses used for collimation in more detail.

Download Full Size | PDF

The oscillator is a monolithic distributed Bragg reflector (DBR) laser emitting at a wavelength of 1056 nm and is optimized for narrow-linewidth operation. We implemented a long resonator with a length of 3 mm in order to increase the cavity Q factor and thus to decrease the linewidth. Furthermore, a longer resonator also decreases the susceptibility of the laser to injection current noise [17]. Longitudinal single mode operation is ensured by a 1 mm long, passive, 6th order Bragg surface grating with a reflectivity of approximately 60%. Lateral single mode operation is achieved by waveguiding with a 4 μm wide ridge waveguide. The rear facet (grating side) is anti-reflection coated to less than 0.1% and the front facet is coated to a reflectivity of 30%. The relatively high front facet reflectivity of 30% makes the laser less susceptible to feedback effects, supports stable laser operation, and increases the cavity Q factor. A detailed analysis of the oscillator can be found in [18].

The two-section amplifier comprises of a preamplifier ridge waveguide entry section and a tapered amplifier section. The ridge waveguide entry section (width = 3μm) serves as a spatial mode filter. The tapered amplifier section has an opening angle of 6° to decrease the optical power density within the amplifier and the intensity at the front facet in order to avoid COD. The active region of both, the oscillator and the amplifier, consists of an InGaAs quantum well structure (MO: triple quantum well, PA: double quantum well) which is embedded in a broad AlGaAs optical confinement layer.

The oscillator and the amplifier chip are mounted with the p-contact up on a gold-coated AlN sub-mount which serves as a heat spreader. The sub-mounts are soldered on a micro-optical bench (footprint 10x50 mm²) which is clamped on a copper heat sink for better handling and contacting. An image of the module including the heat sink is depicted in Fig. 1b. The MO and the PA are contacted separately (no common n-contact) to avoid electrical crosstalk between the current source of the amplifier and the oscillator. The micro-lenses for the beam forming are glued onto the micro-optical bench directly or to rails at the side.

The micro-optical components are positioned by means of a hexapod (F-206.S, Physik Instrumente) with a resolution of 0.3μm. In a first step, the master oscillator is soldered onto the micro-optical bench, the output is collimated, and the micro-isolator is inserted. In a second step the power amplifier is soldered, the remaining lenses are adjusted, and glued.

4. Experimental results

All measurements presented here were carried out with a preamplifier current (I_Pre) of 200 mA and at a temperature of 25°C. The CW (continuous wave) power vs. tapered amplifier section injection current (I_TA) characteristics are shown in Fig. 2 for various oscillator currents (I_MO). The characteristic for I_MO = 50 mA corresponds to that of an unseeded amplifier since the oscillator is driven below threshold (threshold current = 65 mA). Saturation of the amplifier is achieved for I_MO > 150 mA. In the high-power regime (I_TA > 750 mA) a linear slope is observed corresponding to 0.79 W/A. The small signal gain corresponds to 23 dB for I_TA = 2 A.

 

Fig. 2 Dependence of the optical output power of the MOPA module on the injection current into the tapered amplifier section of the amplifier for various oscillator currents. The current through the preamplifier section is 200 mA.

Download Full Size | PDF

The optical spectrum vs. the oscillator current is depicted in Fig. 3 for I_TA = 2000 mA. The laser shows stable single mode operation with only a single mode hop at I_MO = 70 mA. The average wavelength shift is 0.7 nm/A and the side mode suppression ratio exceeds 45 dB. The inset of Fig. 3 shows the optical spectrum of the oscillator in comparison to that of the MOPA for I_MO = 200 mA and I_TA = 2000 mA. The two spectra only differ in a higher ASE background of the MOPA-module which stems from spontaneous emission events within the amplifier. Note, that these spontaneous emission events do not affect the lasing mode and thus do not influence the linewidth owing to the micro-isolator. Except for the excess ASE background, the spectrum of the oscillator is amplified without being modified.

 

Fig. 3 Optical spectrum vs. injection current of the oscillator I_MO for injection currents of I_Pre = 200 mA and I_TA = 2000 mA. The inset shows a single optical spectrum of the oscillator itself at an injection current of 200 mA in comparison with that of the MOPA. The spectra are normalized to the total peak of all emission spectra. In the inset, both spectra are individually normalized to the peak values.

Download Full Size | PDF

The wavelength shift of the entire module with temperature is linear and mode hop free with a slope of 0.07 nm/K, measured for I_MO = 200 mA and I_TA = 2 A. The module can be tuned by 1.4 nm for a temperature swing of 20 K.

Figure 4a shows RF spectra obtained by a heterodyne beat note measurement of two nominally identical MOPA modules for various oscillator current settings. The observed lineshape corresponds to a convolution of two nominal identical optical spectra. The wings of the spectrum are most adequately described by a Lorentzian lineshape, whereas the center lobe resembles a Gaussian one. This can be explained by the two contributions to the frequency noise spectrum, intrinsic noise and technical noise, as described earlier. Frequency noise spectra for various injection current settings are depicted in Fig. 4b. For carrier offset frequencies below 3 MHz a 1/f noise characteristic is observed, whereas for offset frequencies beyond 7 MHz only the white noise component is present. The level of the white noise component is used to calculate the intrinsic linewidth. The FWHM linewidth and the intrinsic linewidth at various oscillator output powers are depicted in Fig. 4c. The FWHM linewidth is almost constant and corresponds to a value of 100 kHz. The intrinsic linewidth is decreasing with a 1/P_MO-dependence, as theoretically expected. Figure 4d shows that the linewidth of the MOPA is not influenced by the power amplifier.

 

Fig. 4 (a) RF beat note spectra of a heterodyne linewidth measurement for various injection currents through the oscillator (I_TA = 2 A). (b) Frequency noise power spectral density (PSD) of a heterodyne linewidth measurement for various injection currents through the oscillator (I_TA = 2 A). (c) FWHM and intrinsic spectral linewidth vs. the optical output power of the oscillator (I_TA = 2 A). (d) FWHM and intrinsic spectral linewidth vs. the injection current through the tapered amplifier section (I_MO = 200 mA).

Download Full Size | PDF

Furthermore, the modules are by far less sensitive to optical feedback from the measurement environment than solitary laser diodes. This is attributed to the fact that the master oscillator is isolated against external optical feedback by the micro-isolator. Depending on the application this may eliminate the necessity for a high-power optical isolator to follow the amplifier output. This is important in terms of saving volume and weight.

5. Conclusion

For the first time, we have demonstrated a narrow-linewidth, high-power, micro-integrated semiconductor based MOPA laser system. The module is implemented on a footprint of 50 x 10 mm² and features a FWHM linewidth of only 100 kHz and an intrinsic linewidth as small as 3.6 kHz for an output power beyond 1 W. To our knowledge, these are the best results for narrow-linewidth, high-power, semiconductor-based laser systems presented in the literature so far. Furthermore, it is the first presentation of a micro-integrated MOPA concept that addresses both, suitability for space application and provision of narrow-linewidth, high-power emission. The module has the potential of replacing larger volume, less efficient, and mechanically less stable narrow-linewidth high-power laser systems used up to date. Although, we demonstrated a device that emits at a wavelength of 1056 nm this principle can be applied to any other wavelength covered by semiconductor lasers.