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As demonstrated by Sawada et al [77,78], a self-oscillating HBT can function equally

well as an optoelectronic up-converter. The HBT photodetects the intensity modulated optical input and up-converts it by the LO frequency which is generated by the HBT

itself. The frequency o f the up-converted signal can be varied by tuning the HBT

oscillation frequency. There are two ways to change the HBT oscillation frequency and up-conversion frequency optically when the HBT also is simultaneously used as an optoelectronic mixer or a microwave mixer. The first is called optical tuning [23] where the frequency o f a free-running HBT oscillator can be changed by the level o f the incident optical power. By changing the incident average optical power, the oscillation frequency can be varied. The other method is called optical injection locking [23]. In this method an optical source is intensity modulated at a frequency which in turn is used to lock the HBT oscillation frequency. When the locking frequency is close enough to the HBT free- running frequency, the HBT oscillator will be locked to the laser modulation frequency. By changing the locking frequency, the HBT oscillation frequency can also be changed and follow the locking frequency, and therefore the up-conversion frequency can be controlled remotely through an optical fibre. Optical injection locking o f a MBSFET self-

oscillating microwave mixer has been demonstrated by Callaghan et al [82]. An injection

locking range o f 8.5 MHz at 6.3 GHz was achieved and a conversion gain o f 1-2 dB for an IF between 50 and 200 MHz was obtained.

It is o f interest to review the current status o f the work on optically controlled HBT oscillators due to their application in optoelectronic mixing.

Bangert et al. [83] were the first to report an optically controlled HBT oscillator. Two

standard GalnP/GaAs HBTs, connected in a differential amplifier configuration, were used to form a simple LC oscillator. One o f the HBTs was illuminated with an 840 nm

optical source above the 10 x 100 jivc^ emitter finger. In the optical tuning experiment,

they achieved a frequency shift from 506 MHz to 481 MHz as the incident optical power was changed from 0 mW to 1 mW, an optical tuning range. A / , o f around 25 MHz. In

locking signal power o f 7.9 mW was provided by a sweep generator to the laser. An optical injection locking range, A/^, o f 15 MHz was achieved with the free-running

oscillation frequency, /q , at 500 MHz (A/. I ~ 3%).

Karakucuk et al. [84] later reported a 2.6 GHz direct optically injection locked

GaAs/AlGaAs HBT oscillator built on a 1 mm thick epoxy substrate using micro strip lines for impedance matching. A 30-period GaAs/AlGaAs multiquantum-well (MQW) was included between the base and the collector, and used to increase the breakdown voltage to 15 V and provided spectral tunability although none o f the mentioned features

were used in the experiments. The circular emitter contact had a 4 jim diameter hole for

optical access to the MQW base-collector depletion region where absorption occurred. With 40 mW (16 dBm) modulation power applied to the AlGaAs laser diode (A, = 830

nm), an optical injection locking range o f around 6 MHz was achieved with the free-

running oscillation frequency at 2.65 GHz, thus A / / « 0.2%. A 5 MHz optical tuning

range was also obtained by changing the incident optical power. In both experiments the

incident optical power was not specified. Karakucuk et al. [85] reported in their next

paper another optically injection locked GaAs/AlGaAs HBT oscillator built on a 15 mil thick Dur o id substrate using micro strip lines for impedance matching. In that work the emitter contact o f the HBT was made o f transparent Indium Tin Oxide (ITO) for improved optical coupling into the HBT. Light was focused onto the transparent ITO emitter contact o f the HBT. With approximately 26 dBm modulation power applied to

the AlGaAs laser diode (X = 830 nm), an optical injection locking range o f 2.5 MHz was

obtained with the oscillator free-running frequency at 6 GHz ( A //^ q = 0.04%). This

small locking range was mainly due to the small optically generated RF power in the HBT which in turn was limited by small laser modulation depth. This RF power was reported to be 30 dB lower than the oscillator output. The optical tuning range was 25 MHz. No information about the incident optical power was given.

Freeman et al. [8 6] extended the frequency o f oscillation with an optically controlled

InAlAs/InGaAs HBT MMIC oscillator to the 14 GHz band with an integrated optical waveguide structure. The oscillator was constructed in a simple feedback configuration consisting o f a planar rectangular inductors and thin film capacitors for the reactive feedback elements. The input optical signal o f 1.55 /im wavelength was first guided

within an integrated optical waveguide running in parallel with and underneath the HBT sub-collector layer. The waveguide had no upper cladding vertically below the HBT so

that the light bent upward and reached the folly depleted collector. The measured

external quantum efficiency was 30 %. The optical tuning experiment showed that a shift o f -100 MHz from the free-running oscillation frequency o f 13.9 GHz was achieved when the input optical power was changed from 0 /xW to 200 ^W. An optical injection locking

range o f only 0.5 MHz was recorded ( A / / - 3 .6 x 10"^% ) due to a very weak optically

generated microwave locking signal power o f -61 dBm in the HBT, compared to a free- running oscillator output power o f -4 dBm.

It can be seen from the review above that optically injection locked oscillators operated at higher frequencies tend to have smaller fractional injection locking range and the reason is believed to be due to the low light coupling efficiency and inefficient direct modulation o f the laser diode at such frequencies.