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What is laser in optical transceivers?

Date: 2019-09-09 15:34 From: C-light Onclick:
What are the role of optical modules in communication equipment?
1. Complete photoelectric conversion and electro-optical conversion; 
2. Realizing the transformation of transmission media (optical fiber - copper wire) of signal; 
3. According to the system requirements, converting a certain quality of optical or electrical signals into signal from the standard definition
 
Laser in Optical transceivers
 
According to OFWeek data, electronic components account for 73% of the cost of optical modules. The electronic components are mainly the optical emission sub-module TOSA for electro-optical conversion and the light-receiving sub-module ROSA for photoelectric conversion, which account for 48% and 32% of the device component cost, respectively.

optical transceiver modules

The main body of the TOSA is a laser chip (VCSEL, DFB, EML, etc.), and the main body of the ROSA is a detector chip (APD, etc.). Optical chips have high technical barriers and are the main cost of device components.

optical transceiver modules C-light 40G optical transceiver module specification

For low-rate optical modules/optical devices (conversion rate less than 10Gbps), the cost of optical chips is about 30%; For high-speed optical modules/optical devices (modulation rate greater than 25Gbps), the cost of chips is about 60%
 
Abbreviation
TOSA: Transmitting Optical Sub-Assembly,
ROSA: Receiving Optical Sub-Assembly,
BOSA: Bi-Directional Optical Sub-Assembly
LD: Laser Diode,
PD: Photo-Diode
 
Semiconductor laser, also known as laser diode, is a semiconductor material as a working material laser. Because of the difference in material structure, the specific process of different kinds of laser is special. Common working substances are gallium arsenide (GaAs), cadmium sulfide (CdS), indium phosphide (InP), zinc sulfide (ZnS) and so on. The excitation modes are electric injection, electron beam excitation and optical pump. Semiconductor laser devices can be divided into homogeneous junction, single heterojunction, double heterojunction and so on. Homogeneous and single heterostructure lasers are mostly pulse devices at room temperature, while double heterostructure lasers can work continuously at room temperature.
 
 
TOSA KPI
 
LD spectral characteristic
Central wavelength(λp) The peak wavelength of all spectral lines emitted by a laser. (nm)
Spectral width(△λ) Half height full width, that is, the spectral width when the light power drops to half of the peak wavelength.(nm)
Spectral shift(△λp) The extent to which the peak wavelength varies with temperature.(nm/℃)
 
LD drive features
Threshold current(Ith) The injection current required to achieve the threshold gain.(mA)
Threshold current drift(△Ith) The extent to which the threshold current varies with temperature.(mA/℃)
Output power(Pf) The output light power of a typical working current.(mW or dBm)
Slope efficiency(SE) The efficiency with which a laser converts current into light power.(mW/mA)
Vf The voltage drop at both ends of the laser tube when the drive current maintains the rated output light power.(V)
Light rise/fall time the time when light power rises from 20% to 80%, and vice versa.(ps)
 
Monitoring photodiode(MD) FEATURES
Backlight current(Im) Monitor photodiode output current at rated output light power.(uA)
Monitor photodiode dark current(Id) Monitor photodiode output current at rated reverse bias voltage.(nA)
Tracking error(TE) Measure the stability of the output light power within the specified operating temperature range.(dB)
 
 
ROSA KPI
 
PD FEATURES
Responsivity(R) The photocurrent produced per unit of input light power.(A/W)
Working wavelength(λ) Effective operating wavelength range
Dark current(Id) PD output current at normal reverse bias without light input.(nA)
Bandwidth(BW) The maximum frequency, or bit rate, that a photodiode can detect without major errors(Hz或bps).
Breakdown voltage(Vbr) The voltage at which the reverse bias of the APD chip reaches saturation.(V)
Gain(M) APD Chip avalanche
 
ROSA FEATURES
Sensitivity(Psen) The minimum optical power that can be detected under the requirement of Specified bit error rate.(dBm)
Saturation power(Psat) The maximum optical power that can be detected under the requirement of specified bit error rate.(dBm)
-3dB bandwidth(BW) The frequency width of an output signal when the amplitude of the output signal is reduced to half the peak value when the input light power is specified.(Hz)
Differential output impedance(Ro) Differential output equivalent resistance.(ohm)
Differential output level(Vp-p) Specifies the TIA differential output level at the input optical power point.(mV)
 
What is Laser?
 
The original full name of Laser is “Light Amplification by Stimulated Emission of Radiation”.
The light emitted by an atom is called a laser: the electrons in the atom absorb energy, jump from a low level to a high level, and then fall back to a low level, releasing energy in the form of photons. A photon beam (laser) is induced (excited) in which the photonic optical properties are highly uniform. This makes the laser more monochromatic, bright and directional than ordinary light sources.
 
Lasers produced by semiconductors emit infrared light that is invisible to our eyes, but their energy is just right to "read" CD and be used in fiber-optic communications
 
Infrared -- wavelength from 10^-3 m to 7.8×10^-7 m;
Visible light -- a very narrow band of light that people can detect. The wavelength is from 780 to 380nm. Light is an electromagnetic wave emitted when electrons in an atom or molecule change their state of motion. It is one of the few electromagnetic waves that we can perceive directly
 
The laser is highly concentrated, which means it has to travel a long distance before it diverges or converges.
 
History of laser
 
In 1951 Charles hard townes, an American physicist, conceived of a way to make radio waves small enough in wavelength by using molecules rather than electronic circuits. Molecules vibrate in a variety of different ways, some of which vibrate exactly as much as radiation in the microwave range. The question is how to convert these vibrations into radiation. In the case of ammonia, it vibrates 24 trillion times a second (24GHz) under the right conditions, so it has the potential to emit microwaves of 1.25 centimetres in length. He envisions pumping energy into ammonia molecules, either thermal or electrical, to "excite" them. Then imagine placing the excited molecules in a microwave beam that has the same natural frequency as the ammonia molecule - a beam that can be very weak in energy. A single ammonia molecule would be subjected to this microwave beam, releasing its energy in the form of a wave of the same wavelength, which in turn would act on another ammonia molecule, causing it to release energy as well. This weak incoming microwave beam ACTS as a starting point for an avalanche, resulting in a strong microwave beam. The energy originally used to excite molecules is converted into a particular kind of radiation.
 
In 1958, American scientists Schawlow and Townes discovered a miracle: when they shone the light from a neon light bulb on a rare earth crystal, the molecules in the crystal gave off a bright, alway converging glare. Based on this phenomenon, they came up with the "laser principle", that is, when a material is excited by the same energy as the natural oscillation frequency of its molecules, it will produce this kind of bright light -- laser. They published important papers and won the Nobel Prize in physics in 1964.
 
On June 5, 2014, NASA used a laser beam to create a 37-second segment called "hello world!" Video took 3.5 seconds, equivalent to a transmission rate of 50 megabits per second, compared with at least 10 minutes for a traditional download.
 
Light
 
Light is a physical term that is essentially a stream of photons at a certain frequency band. The light source emits light because the electrons in the light source gain extra energy. If there isn't enough energy to get it to a more outer orbital, the electrons accelerate and release energy as waves
 
In quantum optics, the energy of light is quantized, and the quanta (basic particles) that make up the light, called "quanta", or photons, can cause chemical changes in materials such as photographic emulsions on film.

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