ECE 183 2012 Lab #3 Tunable laser diode (Agility)

I. OBJECTIVES
ECE 183 2012 Lab #3
Tunable laser diode (Agility)
To study wavelength tuning characteristics of semiconductor laser diodes, and to
become familiar with optical spectrum analyzer and with a modern-day telecom-grade
tunable laser diode.
II. BACKGROUND
Spectral properties of LED and LD
Both LD and LED utilize the emission of light during the recombination of
excited electrons of the conduction band and holes of the valence band of a
semiconductor crystal. When electrical carriers recombine, energy in the form of photons
can be released. This process in the LED is called a spontaneous emission producing
photons in a broad range of wavelengths. In LD the concentration of recombining carriers
is very high and the photons are mostly generated by stimulated emission. In this process,
a photon with certain energy, direction of propagation and phase causes the creation of a
second photon of totally identical properties.
The operation principle of the LD is the same as that for other lasers: the creation of
population inversion that makes stimulated emission more prevalent than absorption.
Both ends of the LD chip are smooth facets acting as mirrors, forming a Fabry-Perot
resonant cavity. This cavity supports several standing waves of different wavelengths,
referred as the longitudinal laser modes.
The fundamental difference between Light Emitting Diode and Laser Diode is
that LD emits very intensive coherent light at few (one or more) discrete and narrow
frequency bands, i.e. resonant longitudinal modes, and LED emits a broad band of
incoherent light over the relatively wide spectral range.
Longitudinal modes of laser resonator.
During laser oscillation, constructive interference allows the creation of a
standing wave within the Fabry-Perot resonator (Fig.1). For light of wavelength λ
travelling in a medium of refractive index n, the half-wavelength in the medium is λ/2n.
As the condition for a standing wave, an integral multiple of the half-wavelength must be
equal to the resonator length L: qλ/2n = L.
Variation of the integer q by 1, causes a wavelength variation by ∆λ. Because of its
relative long length as compared to the light wavelength, the laser resonator may
simultaneously support several standing waves, or longitudinal modes, of slightly
different wavelength (Fig.2).
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Fig.1. Standing wave in laser resonator.
Single-mode Laser Diode
The single-mode emission can be achieved by employing the frequency selecting
element, such as a grating, in order to pick up the desired wavelength from the resonator
modes. One way of generating of such a narrow spectral bandwidth is to place the grating
directly inside the resonator. This solution is known as the distributed feedback, DFB
laser (Fig.3a). Other construction is to place the grating parallel to the junction plane.
Such a single-mode laser is known as the distributed Bragg reflector, DBR (Fig.3b). In
both cases the supplied pump energy is comprised in only one mode. The emitted
wavelength is fixed by the separation of the grating lines and the frequency can be altered
both thermally and by the diode current. The single-mode emission can be also achieved
by the external resonator with grating, the so-called external cavity diode laser, ECDL
(Fig.3c).
Fig.3. Resonators for single-mode lasers
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Fig.2. Output spectrum characteristics

Temperature dependence of the emission wavelength
The laser wavelength increases with increasing temperature. The reason for this is
that the refractive index and the length of the resonator increase with increasing
temperature. Beyond a certain temperature the mode does not fit anymore into the
resonator and another mode which faces more favorable conditions will start to oscillate.
As the distance between two successive modes is very large for the extremely short
resonator (typical 300 µm), the jump is about 1 nm (for λ = 1550nm). Lowering the
temperature gets the laser jumping back in his wavelength. After this the laser must not
be necessarily in the departing mode. Applications anticipating the tuning ability of the
laser diode should therefore be performed within a jump-free range of the characteristic
line (Fig.4).
Fig.4. Emission wavelength as a function of the temperature of the LD and hysteresis.
A similar behavior is observed for the variation of the injection current and in
consequence for the laser output power. Here the change in wavelength is mainly the
result of an increase in the refractive index which again is influenced by the higher
charge density in the active zone. A higher output power provokes also a higher loss of
heat and an increase in temperature of the active zone. The strong dependence of the
current and the output power on the temperature are typical for a semiconductor (Fig.5).
The wavelength of the laser diode depends on the temperature T and the injection
current I in the following way:
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λ(T0, I0) is a known wavelength at T0 and I0.
Generally it is sufficient to consider only the linear terms. For a precision of δλ/λ < 10 -6
the quadratic terms have to be respected. The equation is valid within a jump-free range.
The requirement of λ(T, I)= λc = const provides directly:
Fig.5. Laser power versus injection current with the temperature T as parameter.
Tunable Laser Diode
A tunable laser is a laser the output wavelength of which can be tuned. In some
cases, one wants a wide tuning range, i.e., a wide range of accessible wavelengths, while
in other cases it is sufficient that the laser wavelength can be tuned to a certain value.
Some single-frequency lasers can be continuously tuned over a certain range, while
others can access only discrete wavelengths or at least exhibit mode hopping when being
tuned over a larger range.
Fig.6 shows a schematic of the Sample-Grating, SG-DBR laser. It consists of four
waveguide sections longitudinally integrated together on the semiconductor substrate: a
gain section, containing an active layer embedded within a InP-based semiconductor
waveguide structure; front and back DBR mirror sections, formed by sampled gratings
which have been etched into passive (i.e. not containing active-layer material)
waveguides; and a phase section which contains neither gratings nor active material. All
four sections include electrical contacts. In the case of the gain section, current injection
controls output power. For the other three sections, injected carriers induce a change in
refractive index which tunes the lasing wavelength through shifts in DBR reflectance
spectra and the cavity mode spectrum.
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Fig.6. Schematic diagram of the Sample-Grating Distributed Bragg Reflector laser.
In the SG-DBR architecture, the front and rear mirrors consist of sampled gratings with
multiple reflectivity peaks. By injecting current into these mirrors to change the effective
index of refraction, the peaks may be precisely aligned or misaligned to produce high
reflectivity and high sidemode suppression ratio at the desired wavelength. This tuning
mechanism is used to select a particular cavity mode. By injecting current into the phase
section, the position of the cavity modes may be precisely controlled. It is apparent that a
low magnitude dither applied to the phase section would increase the effective linewidth
without compromising other laser qualities such as power or wavelength. The residual
amplitude modulation is minimal.
Fig.7. Spectrum sampled at ten ITU channels across the tuning range for an SG-DBR
laser.
During the manufacturing process, the laser chip is placed on a carrier, loaded into a
hermetic package, and finally becomes the major component in the completed tunable
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laser assembly (TLA). The TLA includes a microprocessor and other control electronics
to support laser commands (Fig.7). These commands may be accessed using the
evaluation software or directly through the TLA interface.
III. EXPERIMENTS
Fig.7. Evaluation board with mounted SG-DBR laser.
You will be using Anritsu Optical Spectrum Analyzer (OSA), model MS9710B.
Switch-on Procedure of the Optical Spectrum Analyzer.
• Switch-on the line power of OSA.
• Set the spectrum analyzer center wavelength to 1520 nm and the span to 200 nm.
• Set the wavelength resolution to the maximum of 0.07 nm.
• Set the vertical scale to be logarithmic.
A. Spectral Characteristics of LD
In this experiment you will be measuring the optical spectrum of a LD, operating in the
near-infrared (NIR) region with a center wavelength around 1550 nm.
- Use the fiber patch-cord to couple the light from the LD to the OSA.
Note that the fiber connector has notch, and correctly fits into the receptacle in only
one orientation.
- Set ILD current at 5mA and observe the optical spectrum of the emitted light.
- Measure the center wavelength and bandwidth of the output spectrum on -3db (FWHM)
level.
- Set ILD current at 50mA and repeat measurements
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B. Spectral Characteristics of Multimode Fabry-Perot LD.
In this part of the experiment you will be using multimode Fabry-Perot LD from Chorum
Technology, model LDLS-02.
- Using the fiber patch-cord connect LD output with spectrum analyzer input.
- Observe the output spectrum of Multimode LD on the OSA.
Describe what you see.
- Measure the spacing between successive peaks.
- Find the modal frequency spacing ∆ν. (Remember that ∆λ =1nm corresponds ∆ν =
125GHz for central wavelength of 1550nm)
- Calculate the length of the laser resonator. Refractive index of the laser material n = 3.7.
C1. Spectral Characteristics of DFB LD.
In this part of the lab you will be using a DFB Laser Diode, model S3FC1550 from
ThorLabs.
Switch-on Procedure of the DFB Laser Diode:
• Switch-on the line power of the laser diode driver.
• Push “Enable” button to activate the laser light emission.
• Set the temperature controller to 20ºC.
• Set the power output to 1mW.
- Using the fiber patch-cord connect LD output to OSA.
- Set the spectrum analyzer center wavelength to 1550 nm and the span to 5 nm.
You should easily observe a peak in the spectrum with no substantial noise for +/- 1 nm
or so around the peak. At this point center the peak and change the span to 1 nm.
- Measure the center wavelength and bandwidth of the output spectrum on -3db level.
C2. Temperature tuning of DFB laser.
- Set the laser power to 1mW and initial temperature to 15ºC.
- Observe the spectrum. Adjust the central wavelength to move the peak to the left side of
the OSA display.
- Measure the peak wavelength.
- Change LD temperature by 5ºC and observe shifting of the peak.
Note that temperature controller need some time to settle to the new value.
- Continue changing the temperature with 5ºC step until it reaches 40ºC and record the
peak wavelengths.
- Plot λ vs. T. Describe the temperature dependence of the output spectrum. Does the
output wavelength tune smoothly with temperature? What is the wavelength tuning range
of this DFB laser?
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D. Tunable characteristics of SG-DBR Tunable Laser.
In this part of the lab you will be using Agility Tunable Laser Assembly (TLA), model
3105.
- Turn on PC and start Agility Comm. Demonstration application.
- Connect TLA output to OSA.
- Set the center wavelength to 1540 nm and the span to 50 nm.
- At this point, there is no output power, because no channel has been selected. To pick a
channel use the Select Channel box in the upper-left portion of the running application.
- Select a channel and observe the laser output spectrum on the OSA.
- Find the laser tunability range by selecting the first and the last working channels.
Record the respective wavelengths and output power levels. Calculate ∆λ.
- Turn off laser by selecting corresponding command from Send Command box.
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