# High-Power Hybrid Mode-Locked External Cavity Semiconductor Laser Using Tapered Amplifier with Large Tunability

Authors: Andreas Schmitt-Sody; Andreas Velten; Ye Liu; Ladan Arissian; Jean-Claude Diels
Journal: Research Letters in Optics
 (2008)

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## Abstract

We report on hybrid mode-locked laser operation of a tapered semiconductor amplifier in an
external ring cavity, generating pulses as short as 0.5 ps at 88.1 MHz
with an average power of 60 mW.
The mode locking is achieved through a combination of a multiple quantum well
saturable absorber
(>10% modulation depth) and an RF current modulation. This designed
laser has 20 nm tuning bandwidth in continuous wave and 10 nm tuning bandwidth in mode
locking around 786 nm center wavelength at constant temperature.


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## Body

## 1. Introduction

Ultrashort-pulse semiconductor lasers have been
introduced as compact, efficient, low-cost sources for a wide variety of
applications, ranging from nonlinear frequency conversion, remote sensing, and
spectroscopy, to free-space communication. Pioneering work with mode-locked
semiconductor lasers dates back to the early 1980's [1, 2]. Despite two decades of
extensive research on these lasers, their performances have not yet approached
those of crystalline host laser systems in terms of high output power and short
pulse duration. One of the basic challenges is that the small cross-section
area of the standard narrow stripe semiconductor laser limits the power that
can be extracted before laser damage occurs. Therefore the average output power
is limited to only a couple of milliwatts, and the energy typically to 20 pJ
per pulse [3]. Another
difficulty is the complex nonlinear interrelation between the optical field,
the current density, and the index of refraction, resulting in pulse-phase
modulation that can not be easily compensated. Pulse durations range from a few
picoseconds to just under one picosecond, employing intracavity dispersion
compensation. There are two possible solutions to generate high average power
short pulses with semiconductor lasers without any external amplification
elements: (i) use of high average power vertical-external-cavity
surface-emitting (VECSEL) semiconductor lasers in ring or linear cavities
[4] and (ii) use
of tap amplifiers (TAs) in a ring oscillator configuration which is the
approach taken in this paper. Tapered semiconductor amplifiers are laser diodes
with an additional guiding structure confining the beam in the plane of the
semiconductor junction [5, 6]. The basic goal is to avoid saturation and damage due
to high intensities by having the beam expanded throughout its propagation into
the semiconductor material. A variant commonly used today has an index guided
portion for initial amplification, followed by a gain guided expanding
(tapered) region. After leaving the initial waveguide the beam diffracts
practically freely in the dimension along the semiconductor junction. In the
plane perpendicular to the junction it stays confined until it leaves the
semiconductor. Using two cylindrical lenses, it is then possible to create a
high-quality, near circular, collimated, high-power output beam. Tapered
amplifiers are more commonly used in a master-oscillator power amplifier (MOPA)
setup, where the temporal and spatial characteristics of the output of a
low-power oscillator are well maintained through amplification in a tapered
structure [7, 8]. Instead, in the
configuration presented here, the tapered amplifier (Eagleyard, 780 nm, 1 W,
EYP-TPA-0780-01000-3006-CMT03-0000) is used as gain medium in a cavity. The
advantages over the MOPA configuration are ease of alignment, a superior beam
quality mostly defined by the cavity, and cost saving, since only one laser
device is used. It is an unusual mode-locked oscillator for having the highest
gain per cavity roundtrip as compared to most solid-state mode-locked
oscillators. An additional benefit is the unanticipated short pulse duration.
Subpicosecond pulses are generated by hybrid mode-locking without pulse
compression. As compared to  conventional mode-locked semiconductor lasers, the
short pulse duration is achieved at a higher peak power and pulse energy.


## 2. Experimental Results

A commercially available TA is used, with focusing and
collimating optics as well as an isolator.  The optical isolator is required to protect the diode
against any reflection into the large cross-section facet that may result in
laser damage in the small cross-section input end of the device. For testing
purposes, the TA was first characterized in an MOPA
setup, to determine the input intensity (using a Ti:Sapphire laser as a source
at 780 nm) and pump current that lead to a saturated output. The maximum input
power for which no gain saturation is observed, when increasing the pump
current, was determined to be 7 mW. At that input power, the maximum output
power (at maximum pump current) is 610 mW, corresponding to a small signal
optical gain of 87 or 19.4 dB.A sketch of the ring cavity is presented in Figure1,
including the tapered amplifier, and a combination of a half wave plate (HWP1)
and a polarizing beamsplitter (PBS) to limit the power coupled into the input
end of the TA, while extracting the maximum amount of power from the ring
laser. The second half wave plate (HWP2) adjusts the polarization to match the
required input polarization of the TA.Figure 1
Schematic of hybrid mode-locked external ring
cavity semiconductor laser. For mirrorA a normal mirror was used in cw configuration
and a multiple quantum well mirror for mode-locking. The radius of curvature of
both focusing mirrors is 10 cm.In continuous wave operation (with a 100% reflection
mirror at positionA in Figure 1), the output power (measured at
position O in Figure 1) versus current characteristic of
this laser shows a threshold current of 1.1 A, and a spectrum centered at
781 nm, with an FWHM of 0.58 nm at a temperature
setting of 30°C (1.5°C higher than the normal upper limit of the operational temperature
range). The vertical and horizontal beam profiles shown in Figure 2 are seen to
be nearly Gaussian with a M2 of 1.07. It appears thus that the spatial mode
is determined by the external cavity, rather than by the tapered
amplifier.Laser-beam profiles with Gaussian fit in the horizontal (a) and vertical axis (b).
(a)(b)The relation between the TA input and the laser output
(Figure3) was measured by adjusting the output coupling via HWP1 and measuring
the TA input through mirror M1. For this measurement, a multiple quantum well
saturable absorber (MQW) is inserted in position A of Figure 1. The peak output is well below the maximum
possible output of the TA at the applied current and temperature due to the
high losses in the cavity. Those losses result from low-reflectivity mirrors
and the high absorption in the MQW. The measurement is not in the mode-locking
operation, since the MQW modulation solely is not sufficient to start this
mode.Figure 3
Laser output power versus intracavity TA input
power at different pump currents  (+) 2 A,× 1.3 A. The TA input power is monitored behind mirror
M1.Mode-locking is achieved by a combination of a
multiple quantum well saturable absorber, and current modulation at the
cavity repetition rate. Because of the very large gain of the amplifier, it is
possible to use a large number of quantum wells, providing deep modulation. The
MQW saturable absorber is designed with10Al0.1Ga0.9 As layers—a composition that has been designed for saturable
absorption at 795 nm [9]—deposited on top of a Bragg reflector. Double
passage through this structure has a small signal absorption of 20% at the
design wavelength. An RF signal at 88.2 MHz from a standard signal generator is
amplified (Mini Circuit ZHL-1-2W) to a peak-to-peak voltage of 4.5 V, and
applied via a bias-T of 50Ω impedance. The RF frequency is tuned to match the
mode-spacing of the cavity. The mode-locked operation is remarkably less
sensitive to the exact value of the RF frequency and the modulation depth, as
standard actively mode-locked systems such as argon lasers, possibly because of
the low quality factor of the empty cavity. An output power of 40 mW can be extracted from
the polarizing beam splitter (output marked as O in Figure 1). Higher power can readily be
obtained since the TA can deliver >500 mW output, but results in a degradation of
the mode-locking. This is due to the fact that the higher gain is no longer
matched by the nonlinear loss. It should be possible to solve this problem by
increasing the modulation depth of the MQW, as has been done in other high gain
mode-locked lasers, for instance, with 100 quantum wells [10]. Indeed, introducing an
additional MQW of the same composition and 5% modulation leads to 60 mW
(60% increase). In addition it led to the observation of self starting mode
locking without RF modulation at low (<10 mW) output powers. The pump current had to
be adjusted to 1.8 A to compensate for the additional losses. A salient feature
of diode lasers is the short upper state lifetime and low saturation fluence,
characteristics similar to that of dye lasers. As a result, there is no
tendency to Q-switching as is the case in most solid-state lasers (e.g.,
Nd:vanadate, Ti:sapphire, and Cr:LiSAF,). The pure cw mode-locked regime is
verified with a detector-sampling oscilloscope combination (risetime 200 ps), an rf RF spectrum analyzer.The pulses were further characterized by an optical
spectrum analyzer (Ando AQ-6315A). The spectral width was measured to be 2.15 nm with the RF modulation signal applied, as compared to 0.58 nm without RF
modulation (Figure4).Figure 4
Optical spectrum with RF modulation on. Inset,
RF modulation off.A colinear autocorrelation with the exact ratio of 3:1
(Figure5) indicates a well defined subpicosecond pulse [11]. The autocorrelation is
seen to fit a sech2 shape of 0.5 psduration. duration. The
observation of subpicosecond pulses from this laser was unexpected, given that
there is no dispersion compensation. The short pulse duration can be explained
by the large nonlinear attenuation (20% to 30%) of the MQW, as compared to the
fraction of a percent used in conventional solid-state
femtosecond lasers. The use of dispersion compensation should enable us to achieve
shorter pulse durations since it will compensate to some degree the high
self-phase modulation inside the semiconductor [12] and the dispersion from all
the other cavity elements. Reduction of pulse duration by 30% in a
semiconductor laser has previously been reported by using gratings
[13] for dispersion management
inside and outside the cavity. Since TA diodes are
tunable over a range of 20 nm, the spectrum can easily support shorter pulses.
The repetition rate is limited by the bias-T to 100 MHz.Figure 5
Intensity autocorrelation trace of generated
pulse with pump current of 1.3 A.The pulse energy is limited by the comparably short
lifetime of the excited electrons in the TA. It limits the amount of energy
that can be stored in the TA at a given pump current. Since it is on the order
of 1 nanosecond, decreasing the repetition rate below 1 GHz will not result in
an increase in pulse energy. A continuous wave output power of 1 W would thus
lead to a maximum pulse energy of about 1 nJ.


## 3. Tunability of the Laser

Two prism pairs
inserted between the isolator and the MQW were used to test the tunability of
the laser. Wavelength tuning is achieved by translating a 1 mm square aperture
through the beam after the second cavity prism. Tuning curves shown in
Figure6
indicate a cw range of 20 nm, while mode-locked operation is restricted to a 10 nm range. Very limited tuning (about 6 nm) can additionally be achieved by
adjusting the TAs operating temperature between 16°C and 28.5°C. We were not able to get even close the desired wavelength of 795 nm
within that temperature range set by the TA
manufacturer. It should be noted that the wavelength range of the TA, when used
to amplify a tunable master signal, is more than 40 nm according to the
manufacturer specification, and significantly larger than what we observed from
our cavity.Figure 6
Output power versus wavelength for the cw
laser at 2 A (+), 1.3 A (×), and for the mode-locked operation at 1.3 A. The TA
temperature is set at T=28.5°C.


## 4. Conclusion

A novel
external ring cavity hybrid mode-locked semiconductor laser, constructed with a
tapered amplifier, is demonstrated. The high gain of the device makes it
technically possible to introduce a large number of additional elements to
improve its performance. Chirp compensation with gratings, prism, or coatings,
and an MQW with a larger modulation depth are examples. Hybrid mode-locking is
achieved by combining RF modulation with MQW saturable absorber, resulting in a
minimum pulse length of 0.5 picosecond and an average power as high as 60 mW.
The peak power of 1.4 kW, and total pulse energy above 0.68 nJ represent more
than an order of magnitude improvement over those produced by conventional
narrow stripe mode-locked semiconductor lasers [13].These properties, along with the stability,
robustness, beam spatial quality, and ease to align, make this system a
promising replacement for applications requiring high power [14, 15] and where measurements are
performed intracavity [16]. The amplifier and MQW saturable
absorber can be tailored to any wavelength in the visible-near IR range
[9]. The high gain per
round trip makes this an unusual and interesting mode-locked cavity. Further
study, for example, of the effect of a change in the order of cavity elements
could lead to a better understanding of mode-locked laser cavities in general.



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*Source: 865092-2008-07-14.xml*