Understanding The Working Principle Of Optical Modules In One Article

What is an optical module?

SFP, small pluggable optical module. SFP transceiver is small in size, easy to hot-swap, supports SFF8472 standard, easy to read analog quantity (IIC reading), and has high detection accuracy (within +/-2dBm).

The scale of telecommunications market and data communication market continues to increase, and the demand for optical modules will continue to increase. Specific application areas include: Data center: Real-time and massive information interaction between data centers has created the demand for data center Internet networks, and optical fiber communication has become a necessary means to achieve interconnection. Mobile communication base station: The operator's mobile communication base station needs optical modules to realize the interconnection between devices. The base station has RRU and BBU equipment. In the application, we need optical modules and optical fiber jumpers to connect the links of these two devices.

Passive wavelength division system: Passive wavelength division is the highest proportion of the technical solutions adopted by 5G fronthaul, and the passive wavelength division system consists of color optical modules, multiplexers and optical fibers. SANINAS storage network: As a data storage system, NAS (attached network storage device) and SAN (area storage network) both need optical modules.

Let me tell you the internal structure of the SFP optical module

The main parts of the optical module are composed of the optical transmitter component TOSA, the laser driver, the optical receiver component ROSA (the optical receiver part of the L16.2 optical module uses an APD receiver and also requires a boost circuit), a limiting amplifier and a controller.

01 Optical transmitter component TOSA (Transmitter Optical Sub-Assembly) There are two common types of optical transmitter components, one is the TOSA using a light-emitting diode LED package, and the other is the TOSA using a semiconductor laser diode LD package. The former has a wide spectrum line and low coupling efficiency (although the LED can emit a few milliwatts of light power, it has poor directivity, and the part that can be coupled to the optical fiber for transmission only accounts for 1%-2%). However, it is low in price and has a long service life. It is still used in small quantities in low-speed and short-distance situations. It is often used for short-distance data transmission in 100M Ethernet multimode optical fibers, and the wavelength is generally 1300nm. The optical modules we come into contact with generally use laser diodes.

Types of lasers:

1. VCSEL laser (vertical cavity emitting laser): 850nm wavelength, used for short-distance transmission of Gigabit Ethernet multimode fiber. Gigabit Ethernet switches use this type of optical module extensively, and transmission optical boards will not be used, so they are not introduced in detail;

2. FP and DFB lasers.

The difference between them lies in the different output light characteristics. FP laser is a multi-longitudinal mode laser MLM, which can produce light containing several discrete wavelengths. In addition to the main mode of the central wavelength, the sub-modes of other wavelengths also have a high amplitude, and the main mode and sub-mode are also in dynamic competition, but the frequency band is very narrow.

DFB laser is a single longitudinal mode laser SLM, and the main mode optical power accounts for more than 99% of the total luminous power, and other small sub-modes can be ignored.

For these two different types of laser optical modules, the method of testing their spectrum width with a spectrometer is different.

For FP laser optical module, the test of the spectral width on the sending side is to test the RMS spectral width;

For DFB laser optical module, the test of the spectral width on the sending side is to test the -20dB spectral width, and the side mode suppression ratio is required to be tested.

Among the optical modules currently used, the emission wavelength of 155M and 622M modules is 1310nm, and both use FP lasers, while the wavelength of 1550nm uses DFB lasers. Except for 2Km, that is, I-16, which uses FP lasers, all other 2.5G modules use DFB lasers.

The resonant cavity of the laser diode has two reflective mirrors, which are translucent. On the one hand, they form a resonant cavity to ensure that photons move back and forth in it to emit new photons, and on the other hand, a considerable part of the photons are transmitted from the reflector, that is, emit light. The light transmitted from the front mirror is called the main light, which becomes useful transmission in the optical fiber through coupling with the optical fiber.

The light radiated from the rear reflector is called the secondary light, also known as the backlight. TOSA converts the backlight into backlight current, which can be used to monitor the luminous power of the light source device.

Changes in laser input current and output optical power when the temperature rises.

When the optical gain in the laser cavity exceeds the loss of the reflective surface at the cavity end, the laser will emit a coherent optical signal. The current in the laser at the critical point is called the threshold current (lth). As the temperature rises, the optical gain in the laser cavity will decrease. Due to the decrease in the optical gain in the cavity, the laser needs a larger injection current to obtain coherent light output, resulting in an increase in the threshold current of the laser.

Due to the increase in the threshold current, the output optical power decreases. If the optical power is to be kept constant, the driver must output a larger bias current.

In order to compensate for the change in the laser threshold, an "automatic power control (APC)" circuit is required. The APC circuit monitors the laser backlight current and maintains the stability of the optical current by adjusting the laser bias current.

Generally speaking, the proportional relationship between the backlight current and the average optical power is linear. Therefore, by keeping the backlight current stable, the average optical power of the laser is kept constant.

As the temperature rises, the slope of the characteristic curve of the laser input current and output optical power will become smaller, that is, the efficiency of the laser photoelectric conversion is reduced. We know that the extinction ratio Er = 10xIg[P1/PO](B), where P1 and P0 represent the output optical power of the laser when the digital logic signal is "1" and "0", respectively, and P1-PO represents the amplitude of the optical signal after modulation.

Assuming that the output optical power remains unchanged, the reduction of the conversion slope will cause the extinction ratio of the output optical signal to decrease, which will be reflected in the eye diagram, and the opening of the eye diagram will become smaller.

For optical modules, in addition to maintaining the stability of the output optical power, the extinction ratio must also be maintained during temperature changes.

To maintain the stability of the extinction ratio, the modulation current must be increased. The most common method is to use the table lookup method, using the digital adjustable potentiometer (resistor) inside the controller to maintain the extinction ratio.

The digital potentiometer has a temperature-controlled table of current values. The current values are stored in non-volatile memory as a function of temperature. The temperature range is from -45°C to +95°C, with a step size of 2°C. Using the temperature sensor integrated in the chip, the resistance of this resistor can be automatically adjusted as the temperature changes.

The digital potentiometer is set to reduce the resistance value as the temperature increases. It is connected to the "modulation current setting terminal" of the driver. During the temperature increase, the controller looks up the table according to the measured temperature value and continuously reduces the resistance value of the potentiometer, so that the modulation current increases. In this way, the change in the extinction ratio will be compensated.

Another way to maintain the extinction ratio is the K factor compensation method. The "K factor" compensation feature is added to the laser driver. It increases the modulation current proportionally while the laser bias current increases.

To keep the average optical power stable, the bias current is controlled by the APC circuit. As the bias current increases, the circuit extracts a part of the bias current to adjust the modulation current. In this way, the total modulation current is equal to the original modulation current plus the bias current multiplied by a factor K. This K factor can be set by a resistor connected externally to the driver chip. Since the modulation current increases with the bias current, the extinction ratio can be compensated when the laser temperature changes or the laser ages.

In the controller, H0 and H1 are two digital potentiometers built into the controller. H0 is used to control the modulation current, and H1 is used to control the bias current.

The APC function is integrated inside the driver, but its compensation capability is often limited in a wide range of -40 to 85 degrees, so H1 is used to achieve coarse adjustment, and the APC in the driver achieves more accurate automatic adjustment.

Both digital potentiometers use the table lookup method. The specific resistance value is set by the optical module manufacturer based on the characteristics of TOSA. Often, for TOSAs from different manufacturers or different batches, the resistance value must be re-corrected. In addition, in the above figure:

MON1 is used to detect the value of the bias current;

MON2 is used to detect the output optical power;

MON3- is generally used to detect the received optical power.

These measured values can be obtained by reading the corresponding registers through the !C bus. It is easy to use and has high precision. Most manufacturers can ensure that the precision is controlled within 2dBm, which can effectively avoid the problem of inaccurate analog detection of some single boards.

As can be seen from the above figure, the working principle of the optical module is relatively simple. In addition to maintaining stable optical power and extinction ratio, it is necessary to do a good job of RC matching between the driver and the laser (not shown in the above figure, after the serial 10 ohm sister circuit is limited, it is generally necessary to add an RC circuit to the ground). The quality of the optical port indicators of the optical module is determined by these RCs.

02 ROSA (Receiver Optical Sub-Assembly) ROSA contains photodetector diodes and transimpedance amplifiers (TIA). There are two types of photodetector diodes: PIN diodes and APD avalanche diodes.

APD photodiodes have a multiplication effect, which can generate photocurrents that are dozens or even hundreds of times larger than PIN photodiodes under the same light intensity, which is equivalent to a light amplification effect (actually not true light amplification). Therefore, it can greatly improve the sensitivity of the optical receiver (about 10dB higher than the PIN optical receiver). However, the multiplication effect of APD will also amplify the noise coupled into ROSA, affecting the sensitivity of the receiver. Therefore, the optical module using APD as the receiver needs to handle filtering and other issues.

For the receiver, if the optical power is limited to the overload point or lower than the sensitivity, bit errors or LOF may occur. The overload point of the PIN tube is -3dBm (usually it can reach OdBm), and the APD is 9dBm (usually it can reach -5dBm). For the APD receiver, because its overload power is low, if the receiving power is too large, it may be damaged by breakdown.

Among the optical modules we use, except for the L16.1 and L16.2 optical modules that use APD receivers, the rest use PIN tube receivers.

The receiving side of the transceiver is relatively simple. For 2.5G output, some manufacturers use CML output, and some manufacturers use LVPECL output. You need to pay attention to their datasheet.

Most manufacturers of SFP modules use internal AC coupling, and the module is also equipped with pull-up and pull-down matching, so there is no need to add matching on the side close to the optical module.

For MOD_DEFO (optical module in place), MOD_DEF1 (IIC CocK), MOD_DEFO (IIC Data), LOS (contrary to the definition of SFF, high means no optical input, low means normal. SFF is Signal Detect, SD high means there is an optical signal, low means there is no optical signal), TxFault (transmission failure) must be pulled up on the user side.

When SFP detects an abnormal situation and triggers a protection shutdown, TxFault becomes high and there is no optical output. It must be reset with the Tx_Disable signal.