Laser diodes are widely used in the market, but for outsiders, they are usually referred to as light-emitting diodes. So, is light-emitting diode what we call laser diode? What are the differences between them? How to determine the positive and negative terminals of QSI laser diodes?

In optoelectronic devices, laser diodes (LDs) and light emitting diodes (LEDs) are the two most common types of semiconductor light sources. Although they have similar names, they have significant differences in principles, performance, and applications. At the same time, as a well-known manufacturer in the industry, QSI laser diodes often encounter the problem of "how to judge the positive and negative poles", and this article also answers it for everyone.

1、 The difference between laser diodes and light-emitting diodes

1. Different principles of light emission

LEDs emit light through self emission, and photons are spontaneously emitted, resulting in scattered light.

LD generates photons through stimulated radiation and achieves laser output through a resonant cavity structure, with high coherence of the light.

2. Different beam characteristics

LED light sources have a large divergence angle and uneven distribution of light spots, making it difficult to form precise focusing.

The light output by LD has high directionality and low divergence angle, and can be focused into an extremely fine spot through a lens.

3. Output power and efficiency

LEDs generally have lower power and are used for lighting and indication.

LD can output high power and is widely used in laser communication, medical equipment, and industrial processing.

4. Differences in application fields

LED: display, illumination, signal indication.

LD: laser marking, barcode scanning, fiber optic communication, medical testing, etc.

2、 Positive and negative electrode judgment of QSI laser diode

QSI laser diodes are commonly packaged in two or three pin configurations:

1. Two pin LD (only laser diode)

Long legs are generally positive (LD+)

Short legs are usually negative (LD −, mostly connected to the shell)

2. Tripod LD (with monitoring photodiode PD)

The two legs are the positive and negative poles of the LD, used for power supply.

The other foot is a monitoring photodiode PD used for feedback control.

Judgment method:

Using a multimeter diode test, if there is a conduction voltage of approximately 1.5V~2V between the two pins, it is the positive and negative terminals of the LD.

The only remaining foot is PD.

Under illumination, a weak voltage is generated between the PD and the casing to confirm.

Laser diodes are very fragile and cannot be directly connected to power. They must be driven by a constant current source and protected against static electricity.

The essential difference between LED and LD lies in their different emission mechanisms. LED light diverges, while LD light beam concentrates and has strong coherence.

The positive and negative terminals of QSI laser diodes can be distinguished by pin length, packaging identification, and multimeter testing. The PD pin in the three pin model is only used for monitoring and does not participate in the main circuit power supply.