Vertical External Cavity Surface Emitting Semiconductor Lasers Engineering Essay

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The generation of ultra short pulse with high repetition rate attracts the VECSEL in the field of optical engineering such has high speed optical networks and high data processing. Invent of VECSEL gives much attraction towards it, because it gives precise feature compared to the other techniques while it is used in the imaging technique. But it has some issues like alignment of the external cavity and optical beam coupling, since the VECSEL is activated using optical pumping. Apart from these issues, this method gives distinct properties those are not comparable with the other surface emitting lasers. In this literature review of VECSEL, the principle of operation, distinct characteristics, performance parameters and challenges with an application is discussed and the up to date work done also included.

A VECSEL is a semiconductor laser which has a surface emitting Active region gain, and a laser resonator which is pumped either electrically or optically. The semiconductor gain chip contains an Active region with several Quantum Wells over a distributed Bragg reflector (DBR). The laser resonator is completed by the external cavity mirror which is separated from the active region by few cm apart to get the output in a low divergence, circular, diffraction limited beam of good quality. To get good quality beam and laser mode size, the separation between the external mirror and the active region is varied, and the collection efficiency of the system is improved by using a curved mirror in the external cavity mirror. Output power scaling to multiwatt range is achieved by increasing the laser mode area at high input pump source. The thickness of the semiconductor structure scales about few micrometers and it is mounted on a substrate. A heat sink is attached to the substrate to reduce the thermal gradient of the system. Additional components like saturable absorber for passively Mode-locked surface-emitting lasers, Optical Filter for mono frequency output or wavelength tuning, Optical Clocking and Non-linear crystal for intracavity frequency doubling.

The electrical pump method injects charge carriers which limits the output power to 1W and the usable active area. In the case of optical pumping, uniform distribution of the pump power over the active area is applied which eliminates the free charge carrier injection into the undoped active region. Hence we get a reduction in the optical loss and excess heat dissipation.

vertical external cavity surface-emitting laser (VECSEL)

Fig 1: VECSEL setup

Interband transition occurs in the active region material during optical pumping which has broad pump absorption band and no issue on the wavelength stability of the pump laser. The absorption of the optical pump beam is depending on the material used in the active region. Carriers generated during the optical pumping in the barrier are trapped by the Quantum wells. Carrier concentration and the system temperature affect the Quantum Efficiency, Bandwidth gain, Differential gain and Wavelength gain. Due to heat dissipation, the DBR mirror is much affected by the pump power level and spot size than the active region. This system has Bandwidth gain of few tens of nanometers and has wide applications like generation of green laser diode for tiny projectors, spectroscopy and ultra short pulse generation. The sub section of this review deals about the device principle, fabrication of VECSEL, and the ultra short pulse generation using Passive Mode lock technique.

VECSEL Gain Structure:

The VECSEL gain structure consists of a Bragg Mirror and an Active gain region with several quantum wells, fabricated in a semiconductor using an epitaxial growth process. The choice of material for the fabrication of VECSEL structure should be lattice matched. The 27-period Bragg mirror using AlAs/GaAs stack is grown epitaxial on the substrate. The active region is grown over the DBR, in the GaAs active region 6 quantum wells are formed using InGaAs layers which are compressively strained due to higher lattice constant compared to the GaAs with a spacing of λ/2 by GaAs barrier layers of overall thickness 7λ/2. The quantum wells are formed exactly at the anti nodes of the standing wave generated in the laser system.

Fig 2: (a) Layers of VECSEL - active region and DBR (b) Variation of E-field w.r.t position (Z)

The Total length of the active region and the DBR is about 1µm. The window layer is fully transparent, which allows the optical signal to reach the external cavity mirror and the capping layer is to prevent carrier from diffusing to semiconductor surface where the charge carriers combine non-radiatively.

Principle of Operation:

The VECSEL active area is optical pumped using a diode laser source. The optical coupling efficiency is high and most of the incident beam is absorbed by the active region which results in the Interband transition within the barriers. After absorbing the incident pump power charge carriers is created and are confined in the Quantum Well. The quantum well has a high refractive index compared to the barrier layer which enhances the charge confinement in this region by having lower bandgap energy. The transition of the charge carriers in the quantum well gives a higher order exponential gain on the order of 6000/cm [3]. For a thin active layer (10nm) gives out a higher gain in the surface emitting structure compared to the edge emitting structure. Thus the gain is confined inside the active region and it's expressed as a function of confinement factor, 'γ'

Where E (Zi) - Electric Field magnitude in the ith quantum well.

From Fig 2(b) we can observe the electric field intensity of the standing wave confined in the active region. The VECSEL has a strong spectral filtering introduced by the longitudinal confinement factor which controls the device performance. By adjusting the optical sub cavity developed by the DBR and the air interface, the spectral dependence of the longitudinal confinement factor is controlled [1]. The tight control of the longitudinal confinement factor is obtained by making a sharp resonance in the optical sub cavity which improves the gain of the system at a high budget of cost. As the active region is pumped optically, it starts heating inherently increases the optical thickness about 0.1nm/K which reduces the resonance peak and gain of the system scales down.

High temperature dependence in the gain of the VECSEL system; the gain profile shift as the temperature increase to longer wavelength at a rate of 0.3nm/K. However the effective gain decreases with increase in temperature.

Fig 3: Schematic diagram of a VECSEL

The optical pump power is incident on the active region (Gain Structure) of the VECSEL structure increases, the active region temperature increases, the gain decreases and the population inversion occurs at high transition to compensate the reduction in gain. This increases the power dissipation and cut off the laser operation. In the case of surface emitting laser the spectral filtering effect on the longitudinal confinement factor reduces the thermal run away and controls the device performance. The external resonator is fold with flat or curved mirrors and additional semiconductor saturable absorber mirrors (SESAMs). The output coupler defines a TEM00 cavity mode with the spot size equivalent to the incident beam. Addition of SESAMs in the resonator cavity is to get a passive mode locking.

Fig 4: Output Spectrum of VECSEL

The gain is altered by changing the pump source and during the ON-time the gain exceeds the loss co-efficient and the laser light is produced with high output average power. Thus we get high average output power with good beam quality.

High operating Power:

In order to get high output power and high beam quality, the input pump power need to be raised. Design 1: Kuznetsov M et all developed a system with 13 InGaAs quantum wells in the GaAs substrate and the emission wavelength about 980nm. This device architecture is pumped with a laser diode pump with power 3W at operating wavelength 808nm. Output coupler has transmission efficiency about 4% and the beam output power is 0.69W in the TEM11 mode, 0.52 W in TEM00 mode and 0.37 W coupled to a single-mode fiber [7].

Design 2: Holm et all developed a system with the lattice matched AlGaAs/GaAs material with a lesser emission wavelength 870nm shows a lower differential gain and T0 compared the strained InGaAS/GaAs structure but the output power is lower (0.15W).

These two designs have a resonant sub-cavity formed by the active region during operation which causes an increase in effective gain with the addition of spectral filtering effect. Using anti-resonant short sub-cavity in the active region we can reduce the filtering effect which affects the performance of the device.

The output power characteristics of the Design 1 VECSEL device is analyzed by contacting the GaAs substrate with a peltier cooling system by a thermally conductive paste. The device is power scaled upto 1.5W using a fiber coupled laser diode with an emission wavelength of 830nm and spot size 90µm incident on the active region of the VECSEL device and it is operated in the TEM00 mode [5].

Fig 5: Output power spectrum of an InGaAs/GaAs VECSEL pumped with 1.5W laser diode

From the graph it states that at a peltier setting of 0°C the laser output power reaches upto 400mW and roll over abruptly. For 60°C, the laser output power reaches upto 190mW and roll over slowly. It shows that the functionality of the device is limited by the temperature dependence of the quantum well not on the longitudinal confinement factor. The enhancement of the output is done by back reflecting the unabsorbed pump power through the well in the DBR.

There are several ways to reduce the thermal impedance in the device like MOVPE epitaxial grown of DBR and active layer and bonded with the diamond heat sink [7]. An effective method to eliminate the heat from the active region is by placing an uncoated sapphire window on top of the active gain medium. This method enhances the input pump power with less thermal runaway in the device. An alternative material for the heat spreading plate is Silicon Carbide (SiC) which has a thermal conductivity greater than the sapphire material. The advancement of the cooling system in the device results in the invent of microchip VECSEL which has higher thermal impedance tolerance.

Operational Wavelength of VECSEL:

Various wavelength the VECSEL can be operated and with the material systems used for the device structure.

The GaAs substrate is chosen for the near IR operating regime with GaAs/AlAs Bragg Reflector. The material incorporated in the quantum well is a strong function of the operating wavelength. For a device operation wavelength at 850nm, lattice matched GaAs/AlGaAs quantum wells used and at 1000nm strained InGaAs/GaAs quantum wells used. Using an InGaP quantum wells with AlGaInP DBR structure gives an output power of 200mW at emission wavelength of 660nm under argon (green)-ion laser pumping.

In the 1.5µm operating regime InP substrates are used and the DBR mirror stack has higher thickness compared to the GaAs substrate structure due to the long operating wavelength and low refractive index contrast. Absorption and scattering loss are not negligible and the reflector efficiency is reduced. An optimum design with GaInAsP quantum well fabricated with the GaAs/AlAs DBR to overcome the above said issues. This design gives a low electrical impedance and high reflectivity, but it's difficult for manufacturing. Using a GaSb substrate the device can be fabricated to operate in 2 to 2.5µm regime which is used in the atmospheric sensing of pollutants like CH4 and CO.

Performance Parameters:

Gain:

The laser device gain is a strong function of carrier density and it is expressed as,

g=g0ln(N/N0)

As the absorption of the barrier material is increased, which cause more carrier flow in the quantum well and increases the charge confinement results in the higher gain.

Threshold:

Lasing threshold condition is defined as R1R2Tlossexp(2γgthNwLw) = 1

Where R1 and R2 the cavity mirror reflectivity's, Tloss is the transmission factor due to round-trip cavity loss, gth is the threshold material gain, Nw is the number of QW's in the gain medium, and Lw is the QW thickness.

The threshold pump power is dependent on the number of quantum wells. For a small number of quantum wells the threshold pump power increases abruptly and for more quantum wells it increases slowly.

Output laser power increase w.r.t the number of quantum wells and decrease in the reflectivity.

Fig 6: Threshold pump power and output power of input pump source

Output Power:

Laser output power is depend on the input pump power and reducing the reflectivity of the external mirror at higher laser threshold.

Fig 7: Output power of VECSEL

Challenges:

Strain Adjustment:

In order to get high gain, low VT and high output power we need to use a highly lattice matched active gain element in the VECSEL. The semiconductor material strained InGaAs Quantum wells gives promising results but it induces the strain in the device which will affect the structure stability, reliability and performance factor. By adding a tensile strained GaAsP strain compensating layer we can eliminate the issues. The repose curve of tensile strain, bandwidth w.r.t GaAsP composition is shown below,

Fig 8: Tensile strain and bandgap wavelength response of the VECSEL vs. Concentration of GaAsP

Thermal Effect:

In the VECSEL device dissipates heat during high power operation and it should be removed in order to maintain the performance of the system. Mounting the device with the heat sink like diamond, indium and sapphire heat spreading material can reduce the heat. Most of the heat is from the DBR while absorping the optical pump beam and the bottom substrate contributes on the effective heat dissipation [9]. By mounting the DBR directly on the heat sink or by reducing the number of mirror layer we can substantially reduce the heat. Due to the increase in the temperature the spectral shift occurs, reduces the bandwidth, PL shift to longer wavelength and increases refractive index . The output response curve is shown below,

Fig 9: Thermal Impedance of the VECSEL structure Vs chip thickness

Advantages:

Variety of wavelength is generated efficiently using the mature semiconductor material.

Low threshold voltage (VT), High output power, High selectivity and performance efficiency is achieved by Band Gap engineering in the active gain region - quantum well.

Broad bandwidth is achieved by tuning the design parameter.

Operating wavelength of the laser and the pumping source can be selected by the design parameters.

High availability of broad pump bandwidth (40nm) adds an efficient absorption of the input pump beam.

Very short pump absorption length gives a good absorption and good overlap between the laser modes and the input pump beam.

Wide wavelength from UV to IR range can be generated with the existing semiconductor materials.

No p-n junction is implemented in the structure, so easy in batch production and increased reliability of the device.

Since optically pumped no I2R loss.

High output power generation is possible in the wide operating wavelength regime and has wide application like ultra short pulse, intra cavity doubling and spectroscopy.

Disadvantages:

Requires an external pump and external cavity to achieve high output power and requires a stable mechanical design of the laser fabrication.

Size of the VECSEL laser is bigger compared to the conventional laser system.

Applications:

In the semiconductor quantum well surface emitting laser has a broad and homogeneous gain and a wide selection of the center wavelength creates new avenues in the area of ultra short pulse, spectroscopy and output wavelength tunable function.

Intra Cavity laser absorption spectroscopy

Intra Cavity Doubling

Ultra Short Pulse Generation

VECSEL as a Passive Mode locking Surface Emitting Laser to generate ultra short pulse

The broad bandwidth of VESEL output gives more interest in generation of the ultra short pulse using a passive mode lock technique [1]. Incorporating an intracavity semiconductor saturable absorber mirrors (SESAMs) in the external cavity of the surface emitting semiconductor laser we can passively mode lock to generate an ultra short pulse. The device is grown epitaxially with an active region consists of InGaAs/GaAs strained quantum wells on a DBR and it is pumped optically using a high brightness laser diode light source.

In the synchronous pumping of the laser source, with Nd:YAG and titanium sapphire lasers gives a low repetition rate about 100MHz. Thus in VECSEL the continuous pumping of the laser source, the laser cavity is long with round trip frequency about 168MHz and the mode locked occurred at 336MHz (Second Harmonic Oscillation). Because of the longer round trip time the Amplified spontaneous emission reduces the gain and the output power is reduced concurrently and the pulse duration is increased. Using SESAMs in the external cavity of VECSEL we can passively mode lock to generate an effective ultra short pulse [2]. In this review I have discussed about a VESEL structure which generated the output beam with these parameters, pulses of 22ps FWHM at 1030nm with repetition rate about 4.4 GHz.

The optically pumped VECSEL gives an output beam with high average output power and circular diffraction limited with good quality [2]. The output power scaled up to few watts by increasing the input pump source and the laser mode in operation. VECSEL system is initially pumped using an optical pump source - high brightness diode laser. Semiconductor saturable absorbers mirror (SESAMs) in the external cavity is mounted to get a mode lock. The repetition rate of the output beam plays an important role in high-energy physics, optical clocking, and A/D converters. The Nd: YVO has been passively mode locked with pulse repetition rates about 30GHz, pulses - 6.8ps and FWHM at 1064nm. To get ultra short pulse in Nd-doped glass in the range of femto second, due to the large fluence value reduces the reflectivity and the gain saturation is large which results in the Q-switching instability. In order to get low gain saturation with repetition rate of several GHz with no Q-switching, the semiconductor lasers with quantum wells are applied for the passive mode locked. Each quantum well in the active region has a gain as a function of its well width. The output power from the mode locked laser diode is in the order of few tens of mill watts. VECSEL has a peculiar behaviour scaling output power to higher value made much attention towards it.

As shown in the Figure 5, initially the VECSEL device is pumped with laser diode pump emitting wavelength 810nm and output power 1.6W continuously over an active area of 90x90 µm2. The device absorbs 60% of the incident pump source. The optically excited substrate and the SESAM formed the end mirror of the resonant cavity. An output coupler with a radius of curvature 10 mm with transmission efficiency 0.4% of the generated laser wavelength gives an output TEM00 cavity mode (small mode) with the spot size equivalent to the incident beam. If the incident pump source gives a fixed spot size in the gain medium, the by varying the length of the cavity we can change the mode size in the saturable absorber maintaining fixed spot size in the gain medium.

Fig 10: Mode-locked diode-pumped VECSEL cavity with SESAM

The active gain structure is grown epitaxial using MOCVD process with an array of 12 InGaAs compressively strained quantum wells between the GaAsP tensile strained barriers and the thickness of the stack is varied to meet the net strain to zero. The spacing between the quantum wells are maintained λ/2 by GaAs barrier layers. Below the quantum wells, the Bragg mirror is grown epitaxial 27 alternating layers of AlAs and AlGaAs. On top of the quantum well a window layer of about 450nm is patterned to make the carriers away from the surface. Finally a capping layer of 10nm GaAs is grown to complete the VECSEL device structure.

A die area of about 5mm square is diced from the wafer, lapped and polished to reduce the thickness of the GaAs substrate to about 200µm. This device has a capability of generating more heat, so by mounting the die which is soldered with copper and mounted on the indium which acts as a heat sink. Analyzing the photoluminescence spectrum of this device shows a peak at 980nm and lasing occurs in the wavelength range 1000nm to 1040nm as a function of the temperature fluence in the device by pump source and the change in optical thickness across the wafer. A mirror with an incorporated saturable absorber is SESAM processed by the semiconductor technology used for the generation of ultra short pulse by passive mode lock. The SESAM contains GaAs substrate on top of it 25 layer pairs of GaAs/AlAs Bragg mirror is grown. The material used for the Bragg mirror has higher bandgap energy to avoid absorption. A single quantum well of thickness 20nm using InGaAs with a low finesse anti resonant cavity λ/2 is grown at low temperature by Molecular beam epitaxy (MBE) [2]. To get a high modulation depth the thickness of saturable absorber is increased.

The depth of penetration of the optical intensity in the SESAM is a strong function of the location of the saturable absorber material. The key parameters of SESAM are modulation depth, saturation fluence, recovery time and non-saturable losses.

The intensity loss of the SESAM is 1.3% with a recovery time 4ps and the 130fs fast component bleaching response. The length of the active layer is 28mm and the SESAM length is 6mm. Due to short length of the SESAM (40 times lesser than active layer) the cavity mode is tightly focused. This make the absorber saturation pulse shorter than the gain saturation pulse results in fast absorption saturation and fine pulse shaping.

refractive index profile and intensity distribution in a SESAM

Fig 11 (a): Structure of a typical SESAM Fig 11 (b): SESAM - Optical Intensity Profile

The saturable absorber is a quantum well as shown in the figure 6. Instead of a quantum well, if a quantum dot is fabricated on the DBR mirror at high temperature for about 14 layers with a density of 5E10 dots per layer gives a broad absorption profile and uniformity in the spectral response [5]. In this way the saturable loss is reduced down to 1% compared to the quantum well saturable absorber. Hence the saturation fluence magnitude is less and the output response of 13ps from the InGaAs/GaAs VECSEL emission at 1030nm. The slow temperature MBE process of the saturable absorber in the SESAM results in the increase of the saturable loss due to fast charge trap and absorption recovery.

Device Principle:

Once the optical pump with a power of 1.4W is incident on the active layer of the VECSEL structure, the pump source is absorbed by the quantum well. Then the charges excited results in the population inversion.

Fig 12: Excitation and relaxation of carriers in a semiconductor

excitation and relaxation of a SESAM

The Bottom DBR and the external mirror form the cavity which enhances the output beam intensity and quality with output power of 21.6mW - high average output power, the pulses of 22ps FWHM at 1030nm with repetition rate about 4.4 GHz is obtained. Illumination of the pump source on the active region increases the temperature of the device and degrades the gain of the system. The output emission laser wavelength is a function of temperature induced by the pump source and the variation in the optical thickness in the device. Thus increase in the pump source power reduces the output power due to heat dissipation. Several ways explored to scale the output power to higher value with better thermal budget. In the InGaAs/GaAs VECSEL emission at 963nm delivers an average output power of 200mW with near transform limited pulse width FWHM 3.2ps duration [3]. Heat dissipation rose due to the increase in input pump power, but the heat dissipation is reduced by placing an uncoated sapphire window on the active layer which spreads out the heat in the surface. In this way we can raise the input pump power to 7.4W from an 805nm laser diode without any lose to the gain of the system. From a laser operating at 952nm an average output power of 950mW with repetition rate of 6GHz and pulse width of 15ps is produced. The laser is pumped with an input pump source power 15.8W [4]. At the maximum input pump power, the average output power achieved is 2W with a pulse width of 15.3ps and an optical bandwidth of 1nm.

Experimental Results:

Repetition Rate:

The output beam from the VECSEL laser in analyzed using a fast diode monitoring system with a 50-GHz photodiode and a 26-GHz amplifier and spectrum analyzer which gives a radio frequency spectrum contains fundamental peak of the cavity round trip repetition rate about 4.43GHz. From the curve it states that the VECSEL output is stable with no Q-switching and no side band up to the level of -70dBc.

Fig 13. Radio-frequency spectrum of the VECSEL power output showing mode locking without Q-switching instabilities at a repetition rate of 4.43 GHz.

Pulse Duration:

The pulse autocorrelation is carried for the output beam with a best fit which gives a FWHM pulse duration about 22ps.

Fig 14. Autocorrelation trace of the mode-locked pulses

Bandwidth:

The bandwidth of 0.25nm wavelength is shown in the optical spectrum of the VECSEL. The waveform is consistent with no spectral range due to the etalon formed by the DBR and the back surface of the GaAs wafer which is soldered with copper and mounted on the indium. The net saturable absorption loss is about 1% and it gives an asymmetric output response.

Fig 15: Bandwidth spectrum of VECSEL

In the continuous mode operation the effect of etalon shifts the spectral bandwidth and affects the reflectivity of the DBR mirror. The time to bandwidth product of the pulses shows a strong phase modulation effects and it is 1.5 times greater than the transform limit is due to the effects of the dispersion and saturation in the gain structure and the SESAM.

Pulse Shaping:

The quantum wells have a higher differential gain which saturates the gain. This introduces the phase-modulation and it also shapes the tail of the pulse. Refractive index of the active region is a strong dependent of carrier density in that region [6]. Numerical propagation model by R. Paschotta et al deals about the saturable loss and gain, Phase changes, dispersion and bandwidth of the device. A negative phase shift is formed due to the transform limited output pulse which is compensated by the positive phase shift dispersion. As seen in the figure 8, the rate of non-linear phase change is not uniform in the head and tail edge of the pulse results in asymmetry behaviour.

Conclusion:

VECSEL with an optical pumping mechanism gives a high output power with less output loss. The optical pumping gives uniform carrier distribution over a large area with no electrical loss. Because of this benefit this devices finds wide application like intra cavity doubling, ultra short pulse generation and spectroscopy. Using passively mode locked surface emitting laser system we can generate the pulse in the order of femto second with high average output power of several watts. Several thermal impedance reduction techniques are found to improve the performance of the laser system. In current technology it is possible to generate an output power of 3.3W for a heat sink temperature 20°C in the GaSb dual-chip VECSEL at an emission wavelength of 2.25µm. An output power of 3W was achieved in CW operation at 2.0μm emission wavelength for a heat-sink temperature of 20°C, and up to 6W was obtained when the sample was thermoelectrically cooled to −15°C.  In pulsed operation (200ns pulse length), over 21W of on-time output power at room temperature was measured. The optical quantum efficiency of the devices reached very high values of 45% at room temperature and 55% at −15°C heat-sink temperature [Ref: Fraunhofer Institute, Germany, the Institute of Photonics, Glasgow and LISA Laser Products, Germany ]. 

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