1. Introduction

Semiconductor optical amplifiers (SOAs) are devices used for direct amplification of the optical signals. They have epitaxial structure common with standard Fabry-Pérot laser diodes (LDs),but, similarly to super luminescent diodes (SLDs), they have a design preventing the formation of the resonant cavity. This design includes the use of antireflection coatings or/and bent waveguide cavity geometry.

Earlier studies on SOAs were carried out using GaAs/AlGaAs and later InGaAsP/InP materials for the amplification of the light in the 630 to 1600 nm range [1–4]. The main goal of this work was to fabricate a short-wavelength SOA, which could be used as a booster, pre- and in-line amplifier in fiber optical network systems, wavelength converter, optical multiplexer, optical pulse generator, optical logic and gates, and many more [5].

Despite the fact that III-nitride LDs have reached their maturity, the nitride SOAs are still very novel devices, even as compared to nitride SLDs. The main work on (Al,In)GaNSOAs has been so far carried out by Sony group by combining an (Al,In)GaN optical amplifier with mode-lock laser diode in Master Oscillator Power Amplifier system (MOPA) with the purpose to generate high peak power and ultra-short pulses [6–9]. In all those devices the amplified wavelength was in the range of 400 – 405 nm. Such short and high peak power pulses can be used in a next-generation refractive index cornea surgery [10]. In those studies, SOA were AR/AR coated, 2 to 3 mm long taper waveguide devices with the ridge angled 3.5° or 5° relative to the central axis of the chips.

Our work is motivated by the need of providing the optoelectronic community the single mode, high beam quality light source, for the application as atomic clocks (strontium, magnesium). This application requires around 150 – 200 mW of optical power and strictly single mode operation. In this paper, we report the optoelectrical properties of a double ‘j-shape’ waveguide (Al,In)GaN optical amplifier with a peak emission wavelength of 450 nm. Such optical amplifier could be useful as a basic optical element in a plastic optical fiber (POF) in ‘last-mile communication’ optical network system. Moreover, after combining with distributed feedback laser diode (DFB) in MOPA system, it could be useful in gas sensing or Doppler cooling in optical atomic clocks.

2. Sample fabrication

The investigated devices were grown by MOCVD technique on c-plane GaN substrates. The epitaxial structure is a graded index separate confinement heterostructure (GRIN SCH) type with the linear change of refractive index [11]. The epitaxial structure consists of 2 µm Al_0.025Ga_0.975N:Si starting layer, 800 nm Al_0.075Ga_0.925N:Si bottom cladding layer, 350 nm Al_0.075→0Ga_0.925→1N:Si graded bottom cladding, 10 nm GaN:Si transition layer, 50 nm In_0.04Ga_0.96N:Si bottom waveguide layer, 3 x (5.5 nm GaN QB, 3.5 nm In_0.15Ga_0.85N QW), 3 nm GaN cap layer, 65 nm In_0.04Ga_0.96N top waveguide layer, 2 nm GaN transition layer, 20 nm Al_0.12Ga_0.88N:Mg EBL, 100 nm Al_0→0.05Ga_1→0.95N:Mg graded top cladding layer, 550 nm Al_0.05Ga_0.95N:Mg top cladding layer, and finally 200 nm GaN:Mg sub contact layer. Calculated refractive index profile and transversal mode distribution is shown in Fig. 1(a).

 

Fig. 1 (a) Calculated refractive index profile and transversal mode distribution and (b) top view scheme of a double ‘j-shape’ waveguide geometry device. Yellow stripe represents the ridge waveguide.

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The devices were processed to obtain 2.5 mm long amplifiers with 2.5 µm wide ridge. The shape of the processed waveguide is shown in Fig. 1(b). It has a double ‘j-shape’, which means that the waveguide bends at both of its ends. The virtues of ‘j-shape’ waveguide geometry are described in Kafar et al. [12]. The bend waveguide shape is widely used for super luminescent diode fabrication, as allows to significantly decrease the effective reflectivity of the facets. The precise value of the reflectivity strongly depends on the bend angle. The details of this dependence and calculation process can be found in is [13–16].

In this experiment, the bend angle of the ridge relative to the central axis of the chip is 3°. This angle corresponds to the first minimum in the dependence of the effective facet reflectivity on the incidence angle. It was chosen due to technical aspects of the coupling of the light in the MOPA system. To achieve sufficient coupling efficiency of the laser light into the SOA, the amplifier with bent-waveguide geometry has to be tilted with the angle about two times larger than the angle of the bend of the waveguide. Too large rotation can cause difficulty in selecting an objective with a suitable numerical aperture and focal length for the optical pumping. For further decrease of the reflectivity of facets, the amplifiers were coated with anti-reflective coatings (R < 1%) on both mirrors. For reference purpose, also laser diodes were processed from this structure.

3. Experimental results

The measured light vs current dependence of the amplifiers is shown in Fig. 2(a) and the spectra measured for different driving currents are shown in Fig. 2(b). The measurements were performed under continuous wave (CW) operation in room temperature (RT) stabilized by a thermoelectric cooler.

 

Fig. 2 (a) Optical power (per facet) and voltage versus current dependence of investigated SOA and (b) emission spectra measured for different driving currents.

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It can be clearly seen, that below 400 mA the investigated device has exponential dependence of optical power on current which is characteristic for amplifiers or super luminescent diodes. Above 400 mA the dependence changes into a linear regime resulting from a gain saturation. Figure 2(b) proves that the linear regime of L-I characteristic does not mean that the device is lasing – for 500 mA (green, top spectrum) the spectrum is wide, without significant or dominating ripples. The relatively high voltage results from the not optimized die bonding procedure (applied pressure and bonding time) for such large chips (2.5 x 0.4 mm). The improvement of the IV characteristics is expected after the optimization of the die bonding procedure.

Before discussing the experimental results concerning the amplification, the polarization status needs to be analyzed. All investigated devices were grown on c-plane and the mirrors were cleaved along m-plane. It is expected that the light emitted from those devices will be highly polarized. Figure 3 shows the measured polarization state of the emitted light from pumping laser diode and SOA. It can be clearly seen that both devices are highly TE polarized (TE:TM ~200:1). Therefore, all discussed results in this paper concerns TE polarized matched amplification. Although, the polarization sensitivity is unknown, strong polarization dependence should be expected.

 

Fig. 3 Intensity as a function of polarization angle of pumping laser diode (black squares) and investigated SOA (red triangles).

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To investigate the amplification properties of the SOAs we developed MOPA system in which the light from laser diode is coupled into the SOA by NIKON 50x, 0.45 NA objective with working distance 17 mm and collected/collimated by Newport 40x, 0.55 NA plano-convex aspheric lens with 4.5 mm working distance. The optical power was measured by ThorLabs S121C photodiode sensors connected to ThorLabs PM320E power meter. The optical power of the pumping laser diode was measured before the focusing objective and after the neutral density filters and isolator. The optical power of SOA was measured after the collimating lens. Additionally, the spectra of the pumping laser diode, SOA and SOA with coupled laser diode was measured by Horiba FHR1000, 1 m spectrometer with 3600 g/mm grating and Syncerity Deep Cooled 2048x512 pixels CCD camera, providing spectral resolution below 10 pm.

The investigated SOAs and pumping LDs were driven in continuous wave operation (CW) in 20°C ambient temperature, stabilized by a thermoelectric cooler. The SOA output optical power as a function of input laser power is shown in Fig. 4(a) and net gain values as a function of input laser power is shown in Fig. 4(b). It should be noted that the gain values include the input coupling losses (88%). The high resolution spectra were used to separate the amplified spontaneous emission (ASE) and amplified component. The high coupling losses are mainly due to used optics and very narrow thickness of the waveguide layers (more than 10 times thinner than the size of the injected beam). The coupling losses can be reduced by the increase of the thickness of the waveguide layers and by using better optics or fiber. As shown in Fig. 4(b), the gain of 29 dB was obtained for a small signal regime. Figure 5 shows the amplification as a function of SOA output power. For low input laser powers, the SOA output optical power vs input laser power is linear. However, further increase of the input laser power leads to sub-linear increase of the SOA output optical power. This phenomenon, so-called gain saturation, is caused by the fact that the carrier injection rate, which is defined by the current applied to the SOA, sets a limit to the amount of the electron-hole pairs which can possibly recombine per time and by that it limits the rate of generated photons. In other words, under CW operation (constant current) the SOA provides limited amount of the photons/optical power which can be extracted. The input laser power, for which the gain decreases by more than factor of two (3 dB), shows limit for which the SOA, due to the limit of the amount of the injected carriers, cannot longer provide exponential increase of the injected amount of the photons through the length of the SOA. The SOA saturation power can increased by: the increase the active area cross section, the decrease of the confinement factor, the decrease of the carrier lifetime, and finally by the decrease of the differential gain [17].The SOA saturation output power P_sat = 116 mW (20.6 dBm) was obtained for 400 mA SOA’s driving current. The low signal gain and saturation output power values are comparable to the SOAs fabricated with other semiconductor material systems [18–20].

 

Fig. 4 (a) SOA output optical power and (b) calculated net gain values as a function of input laser power for different SOA driving currents.

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Fig. 5 Net gain as a function of SOA output optical power measured for different SOA driving currents.

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Figure 6 shows EL spectrum of pumping laser diode (a), SOA without external laser light (b) and with 33 µW (c), 550 µW (d), 1 mW (e) and 5.5 mW (f) of input laser optical power. It is clear that for low laser optical power signal the spectrum consist of ASE and amplified signal component and with the increasing input laser optical power the ASE is suppressed. At this conditions the amplified light spectrum accurately matches the spectrum of the pumping laser.

 

Fig. 6 Emission spectrum of pumping laser diode (a), SOA driven by a fix current of 300 mA, without external optical signal (b) and with injected laser light power (c–f). The optical power of injected laser light is marked on each spectrum

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4. Conclusions

In this work, we examined the properties of (Al,Ga)N optical amplifier working in MOPA system. To obtain low reflectivity on the facets, and by that to obtain low Fabry-Pérot oscillations, the ‘j-shape’ waveguide was applied together with AR coatings. The bend angle of the waveguide was 3°. For further lowering the reflectivity, the higher bend angle, corresponding to next minimas in the dependence of the effective facet reflectivity on the incidence angle can be applied. However, to successfully couple the light into the amplifier with higher bend angles, the rotation of the chip should be larger, which can be undesirable from the applications point of view and because of mounting problems.

For low input signal, the 29 dB gain was obtained for 400 mA of driving current. The studied SOAs has 3 QW. The saturation output power was 21 dBm. For further increase of the saturation output power, the confinement factor can be decreased by the increase of the thickness of waveguide layers. Moreover, the increase of the thickness of the waveguide layers will lead to the reduction of the coupling losses. However, such approach can result in the decrease of the differential gain, an therefore higher current must be applied to obtain similar optical gain. Last, but not least, to examine the usefulness of the studied (Al,In)GaN optical amplifiers in the plastic optical network system, the dynamic characteristic must be investigated in further studies.