Biomedical Engineering Reference
In-Depth Information
demonstrated by Tai et al. [48] to offer a stable spectral linewidth of 2 GHz and
a stable frequency of operation with fluctuations less than 2 MHz p-p. The
linewidth was stabilized through the use of automatic phase control, while
the laser oscillation frequency was controlled through the use of automatic
injection current control. The basic system shown in Figure 4.22 includes the
following components:
• Laser diode: A GaAs/AlGaAs laser operating at 830 nm and thresh-
old current of 60 mA. The laser is temperature stabilized with a ther-
moelectric element to 13°C ± 0.01°C.
• Mirror: A mirror mounted on a PZT plate 15 cm from the laser is
sued to provide controlled optical feedback. Feedback is also further
controlled through the use of a 1 m fiber-optic ring resonator.
• FORR(1) and FORR(2): A fiber-optic ring resonator constructed with
low loss (0.2 dB) single-mode fiber. FORR(1) also includes a PZT
phase modulator.
• Directional couplers: Directional couplers fabricated by the polish-
ing technique [49].
Linewidth control is provided by the control of the mirror position and
FORR(1). The resonance peak detected by the photodiode(1) and the refer-
ence are compared by the differential amplifier. The output of the differen-
tial amplifier is further processed by an integrator and a buffer amplifier.
The output of the buffer amplifier is then used to control the mirror position
by exciting the PZT plate upon which the mirror is mounted. The cutoff peak
of the FORR(1) and control loop is 1 kHz.
Frequency stability is provided by FORR(2) and the lower control loop.
Frequency fluctuation in the ±12 MHz range is converted into FORR(2)
transmission intensity. The output signal is detected by photodiode(2). The
voltage produced by photodiode(2) is further processed by comparing this
signal with a reference voltage at the differential amplifier and filtering via a
10 kHz low-pass filter. The buffered output of the filter is used to control the
laser diode injection current.
The major drawbacks of this system are the need for external components
and electronic circuitry. The use of many external components must be
weighed against the use of a gas laser where external components may not
be necessary to achieve stable operation.
4.17.10 Rare-Earth-Doped Semiconductor Injection Laser Structures
In an ongoing effort to improve the stability of laser diode operation, manu-
facturers are working to design structures that provide emission frequency
stability over a wider temperature range, provide a decreased linewidth,
eliminate mode hopping, provide a reproducible center frequency from
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