The material LiNbO3 is used in the manufacturing of the following device :
Indirect - Modulator
Lithium niobate is an electro-optic crystal, not a semiconductor. That single fact answers the question. UJTs, LEDs and SCRs are all built from semiconductors — silicon for the UJT and SCR, and III-V compounds such as GaAs or GaN for the LED. LiNbO3 is a ferroelectric oxide, and it is used for its optical properties.
The property that matters — the Pockels effect. Applying an electric field to LiNbO3 changes its refractive index in direct proportion to the field:
\(\Delta n=-\dfrac{1}{2}n^{3}r_{33}E\)
where \(r_{33}\approx31\ \text{pm/V}\) is the electro-optic coefficient. A change of refractive index changes the optical phase along the waveguide, and a Mach–Zehnder interferometer converts that phase change into an amplitude change — giving an optical modulator driven by a voltage.
Why this is called an "indirect" or external modulator. There are two ways to impress data on an optical carrier:
| Direct modulation | Indirect (external) modulation | |
|---|---|---|
| Method | Switch the laser's drive current on and off | Run the laser CW and gate the beam outside it |
| Device | Laser diode alone | LiNbO3 Mach–Zehnder modulator |
| Chirp | Severe — current changes the cavity index and so the wavelength | Very low |
| Speed | limited to a few GHz | 40 GHz and beyond |
Direct modulation is cheap but the wavelength shifts as the current is switched — chirp — and chirp interacts with fibre dispersion to close the eye over long spans. The external LiNbO3 modulator leaves the laser undisturbed, which is why every long-haul and high-bit-rate optical transmitter uses one.
Its other uses. LiNbO3 is also piezoelectric, so it makes surface-acoustic-wave filters, and its high non-linear coefficient is used for second-harmonic generation and optical frequency conversion. A single crystal serving electro-optic, piezoelectric and non-linear roles is what makes it so widely used.
Hence, LiNbO3 is used to make the indirect (external) modulator.
Similar Questions
Match the following :
List – I List – II a. LASER i. Emits monochromatic light of low intensity b. Solar cell ii. Consumes electrical power due to tHe incident light c. Photo diode iii. Delivers power to load d. LED iv. Emits monochromatic light of high intensity Codes :
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1. Ga As
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3. Si
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List - I List - II a) 
i) Solar cell b) 
ii) Pn-photodiode c) 
iii) Pin photodiode d) 
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(a) Homojunctions LEDs are often surface emitters.
(b) Heterojunctions LEDs are Edge emitters.
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Arrange the below mentioned III-V materials to produce LEDs in order of their increasing emission wavelengths :
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Important Questions from Optical Devices
Which computer application scans texts and converts into readable form in computer?
Light wave propagation is possible in optical fibre due to a phenomenon called:
An LCD requires a power of __________.
- The slope efficiency of a laser diode is 0.5 W/A, and the output optical power at a current of 100 mA is 30 mW. Assuming piece-wise linear characteristics of the laser diode, the threshold current of the laser is ________ mA.
- TV remote controls commonly use _____ to communicate with the TV.
