Formation of Hybrid Resonators in the Semiconductor Circular Ring

Laser Diode and Its Output Characteristics

Chun Ting Tsen, Ming Chang Shih and Wen Ho Lan

Department of Electrical Engineering, National University of Kaohsiung, Kaohsiung, R.O.C., Taiwan

Keywords: Semiconductor Laser Diode, Circular Ring Resonator, Optical Nonlinearity, Conjugate Reflection.

Abstract: We present the study of the emission characteristics of a semiconductor circular ring laser diode with a Y-

junction output coupler. Instead of laser output from Y-junction coupling terminal, a confined laser beam

output was also observed at non-waveguide region due to optical nonlinearity effect of the multi-quantum

wells active median. It showed that except for the circular ring resonator mode, much complicate modes were

excited from both output terminals. A hybrid resonator formed by conjugated reflection is suggested to explain

the features of output characteristics. In addition, L-I and spectrum characteristics of both output terminals of

these hybrid resonators were analysed to explore the detailed mechanism of these hybrid resonator.

1 INTRODUCTION

Semiconductor laser diode with circular ring

resonator SCRLD had been attracted interest of

research for its long optical path way advantage of

strong side modes rejection and flexibility to be

integrated with other passive components for the

development of more advanced opto-electronic

systems. Recently, it was demonstrated the control of

CW/CCW mode of the SCRLD output by external

injection light that makes it an ideal all light wave

signal processing system. There are also applications

of SCRLD to achieve wavelength filtering and tuning

of the laser output. However, there is not many

studies about the nonlinearity effect of the SCRLD.

We have studied these devices for decades to show

that instead of linear operation laser mechanism, it is

possible to excited more complex optical nonlinear

effects in these SCRLD devices due to strong

nonlinear coefficient of these multi-quantum wells

active material. We had demonstrated the generation

of Soliton waveguide in a SRCLD with a Y-

junction

output coupler and switching control of light beam

coupling. Here we present the study of the formation

of hybrid resonator due to phase conjugate reflection

in this SCRLD device and its output characteristics.

Figure 1: Material structure and dimensions of the

fabricated SRCLD.

2 FABRICATION OF THE SCRLD

DEVICE

As shown in Fig.1, the fabricated SCRLD was

fabricated with a metal organic chemical vaper

deposition (MOCVD) grown GaInP/InGaAlP

multiple-quantum-well(MQW) active structure with

gain spectrum around 650 nm. Figure 2 shows the

device process flow. First, a SiO

2/Au/Cr layer with

the device pattern was achieved by a lift-off process.

Then, structure of the circular ring resonator with

diameter D = 500



~1000



, depth h= 0.85



~0.9



, width W=10



~25



, and the

Y-junction coupling section was etched by

inductively coupled plasma (ICP) etching.

Tsen, C., Shih, M. and Lan, W.

Formation of Hybrid Resonators in the Semiconductor Circular Ring Laser Diode and Its Output Characteristics.

DOI: 10.5220/0010391301310135

In Proceedings of the 9th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2021), pages 131-135

ISBN: 978-989-758-492-3

131

Figure 2: Device process flow of the SCRLD.

To eliminate the problem of short contact between

the p-n junction, the depth of the ridge waveguide was

etched about 0.8 μm located at just a distances above

the active layer. Secondly, a passivation layer of the

SiO

thin film was deposited using sputtering. Then,

an etch back process was applied to etch out the SiO

layer by using BOE wet etching to open the electrode

area. Then, a blank layer of Au (200 nm)/Cr (10 nm)

was deposited on the SiO

passivation layer. Then,

the substrate was grinded down to a thickness of 150

μm to minimize the resistivity through the substrate.

Finally, an AuGe/Ni alloy thin film was deposited on

the back side for n-type metal contact and then by

annealing the device at 650 °C for 30 s. The fabricated

device was cleaved by a diamond scriber and tested

on a micro probing station.

3 OUTPUT

CHARCTERIZATIONS

The fabricated SCRLD device was probe-tested on a

micro-probing station for light-current (L-I) and

spectrum measurements. Fig. 2 shows the schematics

of the output characterization system in which an HP

8114A pulse power source was operating at 1 kHz for

the current injection on the test device. By using a

beam splitter cube, the output emission can be

measured by the Si-based photodetector for L-I

characterization, and the Jobin Yvon SPEX 500

spectrometer with resolution of 0.01 nm to analyse

the spectrum of the output emission in the same time.

All devices are connected with a PC by GPIB

interfacing for control and data processing. The

output emission from the tested device was

collimated by an object lens (x10) then through a

beam splitter by which to guide part of the emission

to an IR CCD for visualization of the side view of the

emission from the SCRLD.

Figure 3: Schematics of the output characterization system.

Figure 4: CCD image and profile of the emission from

terminal “S”, which shows high order soliton characteristic.

Instead of strong laser emission observed from the

Y-junction ridge waveguide output coupling terminal

(marked as ”Y”), it was also observed a strong and

confined emission at the opposite direction of

terminal Y on non-waveguide region(marked as “S”).

Fig. 4 shows the far field CCD image and profile of

the emission from terminal “S”, which shows high

order soliton characteristic.

We had previously reported that the confined

emission from the non-waveguide region was the

formation of spatial soliton due to the strong 3

order

nonlinearity of the multiple quantum wells medium.

In principle, the spatial solitons formed as the normal

diffraction of the medium can be exactly balanced by

refractive index changes induced by the electro-optic

nonlinear effect. Since the Kerr effect constant of

most of the optical materials is relatively small, a

power density of light on the order of MW/cm

required to form Kerr effect solitons. It was reported

that spatial solitons could be excited by electric-field-

induced refraction index changes in photorefractive

or photovoltaic media. In the case of photorefractive

spatial solitons, the distribution of the field intensity

of a light beam modulates the index of refraction of

the medium via the photorefractive effect, which

compensates for the natural diffraction and achieves

the confinement of the beam profile along the

propagation direction. Photorefractive spatial solitons

can be formed at a relatively lower power of around

100mW that makes them attractive for applications in

PHOTOPTICS 2021 - 9th International Conference on Photonics, Optics and Laser Technology

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integrated optoelectronic devices such as all-optical

light wave control of guiding, switching, and signal

modulation.

Figure 5: L-I measurement of the SRCLD with diameter D=

750 um, the length of Y-junction coupling section L

=1250

um, and the length of non-waveguide section L

=600 um.

Fig. 5 shows the L-I measurements at both terminal

“Y” and terminal “S” of a SCRLD with D=750



W= 20



, h= 0.9



, length of Y-junction

coupling section L

=1250



, and length of soliton

resonator L

=600



. Threshold currents of both

outputs terminal are very close at 0.06 A( ~60 A/cm

Fig. 7a shows the emission spectrum from Y-junction

terminal “Y” with various injection bias. The

emission peak at 652.80 nm is referred to the CW and

CCW propagation mode in the circular ring resonator

and being coupled out through the Y-junction coupler.

As the injection current increase; from 0.1 A to 0.14

A, new spectral peaks located at 650.0 nm and 656.0

nm starting to take off. Peaks position of λ

=652.8 nm,

=650.0 nm, and λ

=656.0 nm are very close to the

relationship of four waves mixing as

321

112





due to 3

order optical nonlinearity excited by

CW/CCW propagation modes in the circular ring

resonator. It is worth to note that λ

=652.8 nm might

referred as the zero dispersion point for this four wave

mixing; FWMX system. Fig. 7b shows the emission

spectrum at soliton terminal “S” that seems close to

the spectrum of terminal “Y” only with a little

deviations of the position between these two

spectrums. In addition to the spectrum measurements,

a polarizer was place at the entrance of the Jobin

Yvon SPEX 500 spectrometer to examine the

polarization of the emission. The result showed that

the emissions from both hybrid resonators were linear

polarized as expected. Fig. 6 shows the measurement

of the polarization scan of the emission from soliton

output terminal.

Figure 6: Polarization scan of the emission from the soliton

output terminal.

Figure 7a: Emission spectrum from the Y-junction

resonator.

Figure 7b: Emission spectrum from the soliton resonator.

Formation of Hybrid Resonators in the Semiconductor Circular Ring Laser Diode and Its Output Characteristics

133

Figure 8: Schematic diagram that described the formation

of the hybrid resonator.

Figure 9a: Emission spectrum at high current injection from

the Y-junction resonator with resonator length L

=1250 um.

Figure 9b: Emission spectrum at high current injection from

the soliton resonator of a length L

=600 um.

At high injection up to 0.16 A, it is shown that

discrete peaks λ

=650.0 nm and λ

=656.0 are

diminishing by taking off a broaden multi-modes

spectrum features located at 654 nm.

A model of hybrid resonator formation is

described in Fig. 8. In the MWQ material with high

order nonlinearity coefficient, phase conjugate

reflection can be excited at the intersection region of

the Y-junction coupling section and the circular ring

resonator by counter propagating modes of E

(CW)

and E

(CCW) in the ring resonator. Light reflected

from the cleaved mirror at the end of the ridge

waveguide Y-junction coupling terminal will be

amplified by phase conjugated reflection and form a

linear resonator. By the same way, another linear

resonator located at the opposite direction of the Y-

junction coupling section can also be formed due to

the confinement of the spatial soliton and

amplification by conjugated reflection. The light

amplification factor is increasing with the intensity of

the ring resonator. In fact, lasing condition of this case

is required by the longitudinal modes of the linear

resonator and the FWMX phase matching condition.

It is explained that spectrum peaks of λ

=650.0

nm and λ

=656.0 are dominating in the emission of

the hybrid resonator. As the injection current keep

increasing to the oscillating point of the conjugate

reflection, the amplification factor of the resonator

drops, as a result the loss of the linear resonator

increase and the emission become broaden. In Fig.5,

a turning point at 0.16 A of the L-I curve is referred

to the reaching of the oscillating point of the

conjugate reflection.

In principle, the circular ring resonator function as

the pumping source for the generation of optical

nonlinearity effect and the outputs modes of the

hybrid resonator are controlled by the optical property

of itself. Fig. 9a shows the detailed spectrum of the

longitudinal modes of the Y-junction resonator of a

length L

=1250



at current injection above 0.16A.

The modes spacing of 0.05 nm is close to the

calculated value using parameter of index of

reflection n=3.6 of the laser medium and the resonator

length of L

=1250



. Fig.9b shows the emission

spectrum of the soliton resonator of a length L

=600



. It shows that the modes spacing of 0.1 nm

which is also very close to calculated value using the

resonator length of L

=600



4 CONCLUSIONS

We have demonstrated that the formation of hybrid

resonators of the SCRLD and its output

characteristics. The conjugated reflection excited by

the strong emission in the circular ring resonator play

an important role to the formation of hybrid

resonators. Since the FWMX is coherent light

interaction in nature thus suggested the laser emission

from the hybrid resonator is coherent, and it is

essential for the development of more advanced opto-

electronic devices applying coherent signal

PHOTOPTICS 2021 - 9th International Conference on Photonics, Optics and Laser Technology

134

processing. Further experimental results to explore

the nonlinearity effect in the SCRLD will be

presented in the near future.

ACKNOWLEDGEMENTS

We would like to thank to Union Optronics

Corporation for their help in device processing and

Ms. W. C. Lin and colleagues for preparing the

experimental data and manuscript. We would also

like to thank the Ministry of Sciences and Technology

in Taiwan for funding this research under the project

number of 107-2221-E-390-014.

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