This set of Optical Communications Multiple Choice Questions & Answers (MCQs) focuses on ” Single Frequency Injection Lasers”.
1. __________________ may be improved through the use of frequency-selective feedback so that the cavity loss is different for various longitudinal modes.
a) Frequency selectivity
b) Longitudinal mode selectivity
c) Electrical feedback
d) Dissipated power
Explanation: Improved longitudinal mode selectivity can be achieved using structures which gives loss discrimination between the desired and all the unwanted modes. Thus, mode discrimination can be seen. To allow for stable mode operation, the use of frequency-selective feedback approach is undertaken.
2. Device which apply the frequency-selective feedback technique to provide single longitudinal operation are referred to as ________________
a) DSM lasers
b) Nd: YAG lasers
c) Glass fiber lasers
d) QD lasers
Explanation: DSM lasers are also known as single frequency lasers. Such devices provide single longitudinal mode operation hence called as dynamic single mode lasers. These lasers reduce fiber intra-modal dispersion within high speed systems.
3. Which of the following does not provide single frequency operation?
a) Short cavity resonator
b) DSM lasers
c) Coupled cavity resonator
d) Fabry-Perot resonator
Explanation: DSM lasers, short cavity resonators, coupled cavity resonators employ frequency selective feedback approach and provide single mode operation. However, the Fabry-Perot resonator allows several longitudinal modes to exist within the gain spectrum of the device.
4. A method for increasing the longitudinal mode discrimination of an injection laser which is commonly used?
a) Decreasing refractive index
b) Increasing the refractive index
c) Increasing cavity length
d) Shortening of cavity length
Explanation: The longitudinal mode discrimination of an injection laser is indirectly proportional to the cavity length. Thus, as the cavity length is shortened, the mode discrimination will get increase. If the cavity length is reduced from 250 to 25 units, the mode spacing is increased from 1 to 10 nm.
5. Conventional cleaved mirror structures are difficult to fabricate with the cavity lengths below __________
a) 200 μm and greater than 150 μm
b) 100 μm and greater than 50 μm
c) 50 μm
d) 150 μm
Explanation: cleaved laser mirrors are used in Fabry-Perot resonator which does not give result for shorter cavity lengths. These lengths may vary from 20 μm to 50μm. Hence micro-cleaved or etched resonator is used for shorter cavity lens.
6. In the given equation, corrugation period is given by lλb/2Ne. If λb is the Bragg wavelength, then what does ‘l’ stand for?
a) Length of cavity
b) Limitation index
c) Integer order of grating
d) Refractive index
Explanation: The period of corrugation is given by
Period of corrugation = lλb/2Ne
Where, λb = Bragg wavelength
L = integer order of grating.
7. The first order grating (l=1) provide the strongest coupling within the device.
Explanation: The period of corrugation is given by lλb/2Ne includes order of grating. The second grating provide larger spatial period and thus helps in fabrication. If the order of grating is 1, then the device is coupled at high level.
8. The semiconductor lasers employing the distributed feedback mechanism are classified in _________________ categories.
Explanation: Considering the device operation, semiconductor lasers are classified into two broad categories referred to as distributed feedback laser and distributed Bragg reflector laser. In the DFB laser, optical grating is applied over the entire active region, whereas in DBR lasers, the grating is etched only near the cavity ends.
9. DBF-BH lasers exhibit low threshold currents in the range of ________________
a) 40 to 50 mA
b) 21 to 30 mA
c) 2 to 5 mA
d) 10 to 20 mA
Explanation: DFB lasers are used to provide single frequency semiconductor optical sources. DFB-BH lasers, developed in laboratories exhibit high modulation speeds, output power but low threshold currents in the range of 10 to 20 mA.
10. Fabry-Perot devices with BH geometries high modulation speeds than DFB-BH lasers.
Explanation: DFB-BH lasers exhibit low threshold currents but high output power and modulation speeds. DFB-BH laser is fabricated by etching or grating. Fabry-perot devices provide modulation speeds of M-bits per seconds whereas, DFB-BH lasers provides modulation speeds of G-bits/sec.
11. The InGaAsP/InP double channel planar DFB-BH laser with a quarter wavelength shifted first order grating provides a single frequency operation and incorporates a phase shift of ______________
a) π/2 Radians
b) 2π Radians
c) π Radians
d) 3π/2 radians
Explanation: The performance of DFB laser is improved by modifying a grating, which in turn introduces an optical phase shift. The phase shift depends on the wavelength used. A quarter wavelength shifted first order grating incorporates the phase shift of π/2 in the corrugation at the center of laser cavity.
12. The narrow line-width obtained under the CW operation for quarter wavelength shifted DFB laser is ________________
a) 2 MHz
b) 10 MHz
c) 3 MHz
d) 1 MHz
Explanation: A quarter wavelength shifted DFB laser provides a large gain difference between the central mode and side modes. It provides improved dynamic single mode stability. Narrow line-width of around 3 MHz can be obtained under CW operation.
13. Line-width narrowing is achieved in DFB lasers by a strategy referred as _______________
a) Noise partition
d) Bragg wavelength detuning
Explanation: Line-width narrowing is achieved in DFB lasers by detuning the lasing wavelength towards the shorter wavelength side of gain peak. It increases the differential gain between the central mode and nearest side mode. This strategy is called as Bragg wavelength detuning.
14. _________________ is a technique used to render the non-conducting material around the active cavity by producing permanent defects in the implanted area.
b) Ion de-plantation
c) Ion implantation
Explanation: Ion implantation approach concentrates the injection current in active region. Current confinement is realized by ion implantation. Ions are implanted into a selective area of a semiconducting material to make it non-conducting.
Sanfoundry Global Education & Learning Series – Optical Communications.
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