What are Laser Diodes?

Catalog

I Laser Diode Parameters

● Wavelength: the working wavelength of the laser tube. The wavelengths of the laser tube that can be used for photoelectric switches are 635nm, 650nm, 670nm, 690nm, 780nm, 810nm, 860nm, 980nm, etc.

High-power laser diode suppliers typically offer standard wavelengths across the spectrum

● Threshold Current(Ith): the current value when the laser tube starts to oscillate. For general low-power laser tubes, the value is about tens of milliamps. The threshold current of a laser tube with a strained multiple quantum well structure can be as low as 10 mA or less.

● Working Current (Iop): the driving current when the laser diode reaches the rated output power. This value is more important for designing and debugging the laser driving circuit.

● Vertical Divergence Angle(θ⊥): The angle at which the light-emitting band of the laser diode spreads in the direction of the vertical PN junction, generally about 15˚-40˚.

● Horizontal Divergence Angle(θ∥): The angle at which the light-emitting band spreads in a direction parallel to the PN junction, which is generally about 6˚-10˚.

● Monitoring Current (Im): the current flowing on the PIN tube when the laser diode is at the rated output power.

Laser diodes have been widely used in low-power photoelectric equipment such as optical disc drives on computers, print heads in laser printers, bar code scanners, laser ranging, laser medical, optical communications, and laser pointers, and are also applied to high-power equipment It has also been used in high-power equipment such as stage lighting, laser surgery, laser welding, and laser weapons.

 Bar Code Scanners

II Working Principle of Laser Diode

A laser diode is a type of semiconductor diode that can generate laser light. The three conditions for generating laser light are: achieving population inversion, meeting threshold conditions, and resonance conditions. Because laser diodes have a very high sensitivity to static electricity, we should pay attention to prevent it during use.

Laser diodes include single heterojunction (SH), double heterojunction (DH), and quantum well (QW) laser diodes. The quantum well laser diode has the advantages of low threshold current and high output power, which is the mainstream product in the market. Compared with optical maser, laser diodes have the advantages of high efficiency, small size, and long life, but their output power is small (generally less than 2mW), linearity and monochromaticity are poor, limiting their applications in cable television systems for not able to transmit multi-channel, high-performance analog signals. In the feedback module of the bidirection optical receiver, the uplink transmission generally uses a quantum well laser diode as a light source.

The basic structure of a semiconductor laser diode is shown in the figure. A pair of parallel planes perpendicular to the PN junction surface constitute a Fabry-Perot resonant cavity.

 Laser Diode Construction

The two planes can be cleaved surfaces of semiconductor crystals or polished surfaces. The other two sides are relatively rough to eliminate the laser effect in other directions except for the main direction.

Light emission in semiconductors usually results from the recombination of carriers. When a forward voltage is applied to the semiconductor PN junction, the PN junction barrier is weakened, forcing electrons into the P region from the N region through the PN junction, and holes are injected into the N region from the P region through the PN junction. The nonequilibrium electrons and holes will recombine with each other and emit a photon with a wavelength of λ. The formula is as follows:

λ = hc / Eg (1).

In the formula:

h—Planck constant;

c—speed of light;

Eg—forbidden bandwidth of semiconductor.

The aforementioned phenomenon of emitting light due to the spontaneous recombination of electrons and holes is called spontaneous emission. When the photons generated by spontaneous radiation pass through the semiconductor, once they have passed the vicinity of the emitted electron-hole pair, they can be excited to recombine and generate new photons. This photon induces the excited carriers to recombine and emit new photons, which is called stimulated radiation.

 the characteristic curve of a laser diode

If the injection current is sufficiently large, a carrier distribution opposite to the thermal equilibrium state is formed, that is, the number of particles is reversed. When a large number of carriers in the active layer are inverted, a small number of photons generated by spontaneous radiation generate induced radiation due to reciprocating reflections at both end faces of the cavity, resulting in the positive feedback of frequency-selective resonance, or the gain for a certain frequency. When the gain is greater than the absorption loss, a coherent light with a good spectral line-a laser, can be emitted from the PN junction. This is the simple principle of a laser diode.

III Luminescence Theory of Laser Diode

The P-N junction in the laser diode is formed by two doped gallium arsenide layers. It has two flat-ended structures, which are paralleled to a mirror image (highly reflective surface) and one partial reflection. The wavelength of the light to be emitted is directly related to the length of the connection.

When the P-N junction is forward bias by an external voltage source, the electrons move through the junction and recombine like a normal diode. When electrons and holes recombine, photons are released and hit the atom, releasing more photons. As the forward bias current increases, more electrons enter the depletion region and more photons are emitted. Eventually, some photons that randomly drift in the depletion region illuminate the reflecting surface vertically and reflect back along their original path. The reflected photons are reflected back from the other end of the junction. This movement of the photon from one end to the other is repeated multiple times.

Depletion Region

During the photon movement, more atoms will release more photons due to the avalanche effect. This process of reflecting and generating more and more photons produces a very intense laser beam. Each photon generated during the emission process explained above is the same as the other photons in energy level, phase relationship, and frequency. Therefore, the emission process gives a single-wavelength laser beam. In order to produce a laser, the current of the laser diode must be above a certain threshold level. Otherwise, the diode will behave as an LED that emitting incoherent light.

IV Physical Structure Performance of a Laser Diode

A layer of semiconductor with photoactivity is placed between the junctions of the light-emitting diodes, and its end surface has a partial reflection function after polishing, so an optical resonant cavity is formed. In the case of forwarding bias, the LED junction emits light that interacts with the optical resonator, thereby further exciting the single-wavelength light emitted from the junction. The physical properties of this light are related to the material.

In the VCD machine, the semiconductor laser diode is one of the core components of the laser head. It is mostly composed of a ternary compound of gallium aluminum arsenic (AsALGA)  with a double heterostructure. The head is a near-infrared semiconductor device with a wavelength of 780 - 820 nm and rated power of 3 - 5 mw. In addition, there is also a visible light (such as red light) semiconductor laser diode, which is also widely used in VCD machines and bar code readers.

The shape and dimensions of the laser diode

The shape and dimensions of the laser diode are shown in Figure 6. There are three types of internal structures, as shown in Figure 7.

As can be seen from Figure 7, the internal structure of the laser diode includes two parts:

(1) a laser emitting part: it can be represented by LD, which is to emit laser light, as the electrode (2) in the figure;

(2) a laser receiving part, represented by PD, which is used to receive and monitor "the laser emitted by JD as shown as an electrode (3) in the figure. If you do not need to monitor the output of LD, this part can be omitted.

These two parts share a common electrode (1 ), So the laser diode has three electrodes.

Three types of laser diode internal structures

V Testing Methods and Precautions

1. Detection of Laser Diodes

(1) Resistance Measurement

Remove the laser diode, and measure the forward and reverse resistance values with the R×1k or R×10k range of the multimeter. Normally, the forward resistance value is between 20 - 40kΩ, and the reverse resistance value is infinity(∞). If the measured forward resistance value has exceeded 50kΩ, the performance of the laser diode has decreased. If the measured forward resistance value is greater than 90kΩ, it means that the diode is severely aged and can no longer be used.

(2) Current Measurement

Use a multimeter to measure the voltage drop across the load resistor in the laser diode drive circuit, and then estimate the current flowing through the tube according to Ohm's law. When the current exceeds 100mA, and we adjust the laser power potentiometers, If there is no obvious change in the current, the laser diode can be seriously aged. If the current increases sharply and gets out of control, the optical cavity of the laser diode is damaged.

2. Matters Needing Attention

(1) The laser emitted by the laser diode may do harm to the human eyes. When the diode is in operation, it is strictly forbidden to look directly at its end face. It is not allowed to look directly at the laser through the lens, nor to observe the laser through the anaglyphoscope.

(2) The device requires a suitable driving source. The instantaneous reverse current cannot exceed 2uA, and the reverse voltage must not exceed 3V, or the device will be damaged. Take effective measures to prevent inrush current when the drive power supply is on and off. When testing the drive circuit with an oscilloscope, we must disconnect the power supply before connecting the oscilloscope probe. If the probe is connected with power applied, the surge current may damage the device.

(3) The device should be stored or operated in a clean environment.

(4) Working at high temperatures will increase the threshold current, lower the conversion frequency, and accelerate the aging of the device. When adjusting the light input, we should use an optical power meter to prevent it from exceeding the rated output.

(5) If the output power is higher than the specified parameters, it will accelerate the aging of the components.

(6) The heart of the machine needs to be fully dissipated or the device must be operated under refrigeration conditions. The temperature of the laser diode is strictly controlled below 20 degrees to ensure its service life.

(7) The diode is a static-sensitive device, which can be taken only when the human body is in good condition. We can wear anti-static bracelets to prevent static electricity.

(8) The output wavelength of the laser is affected by the working current and heat dissipation. It is necessary to maintain good heat dissipation conditions and reduce the temperature of the die during work. Adding a heat sink could prevent the laser diode from overheating during operation.

 A heat sink

VI Laser Diode Drive Circuit Diagram

1. Laser Diode Driving Circuit No1

The automatic power control circuit relies on the internal PIN tube of the laser to detect the output optical power of the LD as feedback. The circuit diagram is shown in Figure 9. In the picture, Dl is a backlight detection diode inside the laser. The current is converted into a voltage by a sampling resistor, and then the voltage is amplified by a differential amplifier. The laser bias current is adjusted by a proportional plus integral controller.

The automatic power control circuit

For a laser with a refrigerator, temperature control is also required, especially for wavelength division multiplexing lasers that require stable wavelengths and an automatic temperature control circuit. The temperature control circuit is shown in Figure 10:

The temperature control circuit

In Figure 10, RZ is a thermistor, R1 is a refrigerator. The forward current of the refrigerator is heating, and the reverse flow is cooling.

2. Laser Diode Driving Circuit No2

(1) Circuit Structure and Principle

LD works by direct carrier injection. The stability of the injected current has a direct and obvious impact on the output of the laser. Therefore, the LD drive power supply needs to provide a steady current with a small ripple and few glitches. The LD driving power supply includes 4 parts: a reference voltage source, a constant current source circuit, a pulse control circuit, and a protection circuit. The block diagram is shown in Figure 11.

Composition of driving power source circuit

(2) Reference Voltage Source Circuit

The structure of the reference voltage source circuit is shown in Figure 12. It can provide a high-accuracy, low-temperature drift voltage reference for the constant current source circuit. At the same time, it provides stable working voltage for integrated circuits, such as optocouplers, operational amplifiers, inverters, etc..

Reference voltage source circuit

(3) Constant current source circuit

In order to achieve high current stability, most drive circuits use a negative feedback control method. The principle of constant current control is shown in Figure 13. The current stabilization circuit consists of a reference voltage circuit, a voltage-current conversion circuit, a constant current output circuit, and a feedback circuit. When the circuit is working, the reference voltage is appropriately amplified and sent to the non-inverting terminal of op-amp A1. Op-amp A1 controls the magnitude of VQ1 base current to obtain the corresponding output current that generates a sampling voltage on the sampling resistor R. After the voltage is amplified by A2, it is fed back to the inverting input of voltage amplifier A1 as a feedback voltage, and is compared with the non-inverting input to adjust the output voltage, and further adjust the output current of the VQ1 base, so that the entire closed-loop feedback system is in a dynamic balance to stabilize the output current.

The principle of constant current control

3. Laser Diode Driving Circuit No3

The simplest driving circuit is a common emitter driving circuit, as shown in Figure 14.

When the digital electrical signal Vi = 0, BG is turned off, and no current flows through the light source device, so it does not emit light (vacant number).

When Vi = 1, the transistor BG is saturated, and a current flows in the light source device and reaches the required working current such as 50MA, so it emits light (marking).

Common emitter driving circuit

In this way, the electrical signal is turned into a light pulse signal, completing the intensity modulation process.

Conclusion

In this article, we mainly discuss the parameters, working principle, luminescence theory, physical structure performance, testing methods, precautions, and 3 drive circuit diagrams of laser diodes. Hope you will enjoy it and find it useful. For more articles about laser diodes, you could click the link below.

Recommended Articles:

How does a Photodiode Work?

Rectifier Diode: Function and Circuit

What are Avalanche Diodes?