Injection Laser:

 Theory

Semiconductor lasers or laser diodes play a significant part in our everyday lives by providing cheap and compact-size lasers. They consist of complex multi-layer structures requiring nanometer-scale accuracy and an elaborate design. Their theoretical description is significant not only from a fundamental point of view but also to generate new and improved designs. It is popular in all systems that the laser is an inverted carrier density system. The carrier inversion leads to an electromagnetic polarization that drives an electric field 𝐸(𝑡). In most cases, the electric field, confined in a resonator, the properties of that are also influencing factors for laser performance. [9]

Gain medium

In semiconductor laser theory, the optical gain is in a semiconductor material. The choice of material depends on the desired wavelength and properties, such as modulation speed. It may be a bulk semiconductor, but more often a quantum heterostructure. Pumping may be electrically or optically (disk laser). We can describe all these structures in a common framework and differing levels of complexity and accuracy. [10]

Light is generated in a semiconductor laser by radiative recombination of electrons and holes.

It consists of a p-n junction formed by two heavily doped semiconductors, with its end faces perpendicular to the plane of the p-n position. The working principle is like that of LED (Light Emitting Diode). The p-n junction is the active medium. Lasing action proceeds after establishing population inversion. 

Injection of minority carriers into the depletion region helps to achieve population inversion. Application of a forward bias voltage (of about 1.5 V) to the active region results in the excitation of electrons from the valency band to the conduction band, leaving a hole behind. Population inversion happens when electrons and holes separate. Laser emission occurs when the recombination of these electrons and holes takes place.
Direct band gap semiconductors like GaAs, PbS, etc., are suitable for the fabrication of injection lasers.

In this article, I discuss injection lasers and some recent developments in solid-state lasers.

Construction:

Two heavily doped semiconductors join together to form a p-n junction. One pair of end faces perpendicular to the Z axis are polished to serve as the reflecting surface of the resonance cavity. The other two end faces (perpendicular to the Y axis) have a rough finish to serve as a partially reflecting surface (Figure 1). A widely used semiconductor for the fabrication of injection lasers is GaAs, and it gives 0.84 µm wavelength.

 

    

                                                                                                                                (b)

(a) Schematic diagram of semiconductor laser

  (b)  Transverse distribution of light intensity      

Working:

The energy band structure of the p-n junction diode, at absolute zero, is shown in Figure 2. The semiconductors are heavily doped. The Fermi level of the p-type semiconductor falls within the valency band. The Fermi level of the n-type semiconductor falls within the conduction band. In the absence of voltage, two Fermi levels lie in the same horizontal line in the specimen. When we apply a voltage (V) to the diode, the two Fermi levels become separated by an amount ΔF=eV. Now, the active region achieves population inversion. Forward biasing leads the electron from the conduction band of the n-type semiconductor and holes from the valency band of the p-type semiconductor into the active region of the diode.

The radiation emission occurs during the process of recombination. The frequency of the emitted radiation is nearly the frequency of the band gap energy.

The active region (1µm) is wider than the depletion region. The transverse dimension of the laser beam is much (nearly 40µm) broader than the width of the active locations. The laser beam thus extends to the p and n regions. Hence, the output beam ends up with a wide beam divergence. 

Energy level diagram of a p-n junction semiconductor laser

(a) zero bias (b) with forward bias (6)

Dependence of Temperature:

An increase in the temperature of the semiconductor leads to an increase in the threshold current, the current required to achieve population inversion. This condition results in higher losses and low gain. The temperature above which the semiconductor fails to give continuous laser is critical. The critical temperature of the GaAs laser is 77 k. Below this temperature, it gives the output up to a few watts and an efficiency of up to 30%.

Advantages:

• Small size
• Small operating voltage
• High efficiency
• Output laser can be modulated easily by varying threshold current
• Unique applications in optical fiber communication.

Electrical and optical pumping

Injection of holes from the p-doped into the n-doped semiconductor, and electrons vice versa. A depletion region, devoid of any charge carriers, forms because of the difference in electrical potential between n- and p-type semiconductors wherever they are in physical contact. Due to the use of charge injection in powering most diode lasers, this class of lasers is sometimes termed injection lasers or injection laser diodes (ILD). As diode lasers are semiconductor devices, they may also be in the category of semiconductor lasers. Either designation distinguishes diode lasers from solid-state lasers.

Another method of powering some diode lasers is the use of optical pumping. Optically pumped semiconductor lasers (OPSL) use an III-V semiconductor chip as the gain medium and another laser (often another diode laser) as the pump source. OPSLs offer several advantages over ILDs, particularly in wavelength selection and lack of interference from internal electrode structures. [11][12] A further advantage of OPSLs is the invariance of the beam parameters – divergence, shape, and pointing – as pump power (and hence output power) is varied, even over a 10:1 output power ratio. [13]

Generation of spontaneous emission

When an electron and a hole are present in the same region, they may recombine or annihilate by producing a spontaneous emission — i.e., the electron may re-occupy the energy state of the hole, emitting a photon with energy equal to the difference between the electron's original state and hole's state. (In a conventional semiconductor junction diode, the energy released from the recombination of electrons and holes is the phonons, i.e., lattice vibrations, rather than as photons.) Spontaneous emission below the lasing threshold produces similar properties to an LED. Spontaneous emission is necessary to initiate laser oscillation, but it is one of the several sources of inefficiency in lasing oscillation. [9]

Direct and indirect bandgap semiconductors

The difference between the photon-emitting semiconductor laser and a conventional phonon-emitting (non-light-emitting) semiconductor junction diode lies in the type of semiconductor used, one whose physical and atomic structure confers the possibility for photon emission. These photon-emitting semiconductors are the so-called direct bandgap semiconductors. The properties of silicon and germanium are single-element semiconductors have bandgaps that do not align in the way needed to allow photon emission. They are considered indirect. Other materials, the so-called compound semiconductors, have virtually identical crystalline structures as silicon or germanium but use alternating arrangements of two different atomic species in a checkerboard-like pattern to break the symmetry. The transition between the materials in the alternating pattern creates the critical direct bandgap property. Gallium arsenide, indium phosphide, gallium antimonide, and gallium nitride are all examples of compound semiconductor materials used to create junction diodes that emit light. [9]

Generation of stimulated emission

In the absence of stimulated emission (e.g., lasing) conditions, electrons and holes may coexist in proximity to one another without recombining, for some time, termed the upper-state lifetime or recombination time (about a nanosecond for typical diode laser materials), before they recombine. A nearby photon with energy equal to the recombination energy can cause recombination by stimulated emission. This stimulated emission generates another photon of the same frequency, polarization, and phase, traveling in the same direction as the first radiation. That means that stimulated emission will cause gain in an optical wave (of the correct wavelength) in the injection region, and the gain increases as the number of electrons and holes injected across the junction increases. The spontaneous and stimulated emission processes are vastly more efficient in direct bandgap semiconductors than in indirect bandgap semiconductors; therefore, silicon is not a common material for laser diodes. [9]

Optical cavity and laser modes

As in other lasers, the gain region is in an optical cavity to form a laser. The two ends of the crystal are cleaved to form perfectly smooth, parallel edges, forming a Fabry–Pérot (FP) resonator. Photons emitted into a mode of the waveguide will travel along the waveguide and be reflected several times from each end face before they exit. As a light wave passes through the cavity, amplification happens by stimulated emission. However, a loss of light due to absorption and incomplete reflection from the end facets occurs. Finally, if there is more amplification than loss, the diode begins to lasing.

The geometry of the optical cavity is a related factor of Some properties of laser diodes. Generally, the light is within a skinny layer, and the structure supports only a single optical mode in the direction perpendicular to the layers. In the transverse direction, if the waveguide is wide enough compared to the wavelength of light, the waveguide can support multiple transverse optical modes, and the laser is known as multi-mode. These transversely multi-mode lasers are adequate in cases where one needs a large amount of power.
In applications, we need a focused beam, and the waveguide must be made narrow, of the order of the optical wavelength. This way, only a single transverse mode is supported, and one ends up with a diffraction-limited beam. Such single spatial mode devices apply for optical storage, laser pointers, and fiber optics. Note that- these lasers may still support multiple longitudinal modes and lase at multiple wavelengths simultaneously. The wavelength emitted is a function of the band gap of the material and modes of the optical cavity. Generally, the maximum gain will occur for photons with energy slightly above the band-gap energy, and the modes nearest the peak of the gain curve will lase most strongly. The width of the gain curve will determine the number of additional side modes that may also lase, depending on the operating conditions. An FP laser will lase at multiple cavity modes within the gain bandwidth of the lasing medium. The number of lasing modes in an FP laser is usually unstable and can fluctuate due to changes in current or temperature.

A packaged laser diode shown with a penny for scale*488 nm: InGaN green-blue laser; became widely available in mid-2018. [9]

Single spatial mode diode lasers can operate on a single longitudinal mode. These single-frequency diode lasers exhibit high degrees of stability and have applications in spectroscopy and metrology and as frequency references. Classification of single-frequency diode lasers is either distributed-feedback (DFB) lasers or distributed Bragg reflector (DBR) lasers. [9]

Formation of laser beam

Due to diffraction, the beam diverges (expands) rapidly after leaving the chip, typically at 30 degrees vertically by 10 degrees laterally. Using a lens, form a collimated beam like that produced by a laser pointer. For obtaining a circular beam, we can use a cylindrical lens. For single spatial mode lasers, using symmetrical lenses, the collimated beam ends up being elliptical due to the difference in the vertical and lateral divergences.

It is easily observable with a red laser pointer. [9]

The simple diode described above has been heavily modified in recent years to accommodate modern technology, resulting in different laser diodes. 

Applications

Laser diodes can produce high-power outputs, continuous waves, or pulsed. Such arrays may efficiently pump solid-state lasers for high-average power drilling, burning, or inertial confinement fusion.

Laser diodes are numerically the most common laser type, with 2004 sales of approximately 733 million units [14] compared to 131,000 other lasers. [15]

Telecommunications, scanning, and spectrometry

Laser diodes are widely used in telecommunication as easily modulated and easily coupled light sources for fiber-optic communication.

Used as :

• Rangefinders.
• Barcode readers. 
• Visible laser pointers.
• Low and high-power diodes are used extensively in the printing industry as light sources for scanning (input) images and high-speed, high-resolution printing plate (output) manufacturing. 
• Infrared and red laser diodes have many uses in CD players, CD-ROMs, and DVD technology. 
• Violet lasers in HD DVD.
• Diode lasers have also found many applications in laser absorption spectrometry (LAS) for high-speed, low-cost assessment or monitoring of the concentration of various species in the gas phase.
• High-power laser diodes in industrial applications for heat treating, cladding, seam welding, and pumping other lasers, such as diode-pumped solid-state lasers.

Many applications of diode lasers use the energy property of an optical beam. In this category, one might include laser printers, barcode readers, image scanning, illuminators, designators, optical data recording, combustion ignition, laser surgery, industrial sorting, industrial machining, wireless power transfer (as power beaming), and directed energy weaponry. Some of these applications are well-established, while others are emerging.

Medical uses

Laser medicine:

Dentistry has found many new uses for diode lasers. [16][17][18][19][20][21] The shrinking size, cost[22], and increasing user-friendliness make it attractive to clinicians for minor soft tissue procedures. Soft tissues hardly absorb diode wavelength ranges from 810 to 1,100 nm and are not used for cutting or ablation. [23][24][25][26] We do not use laser beams to cut soft tissues. We can cut it with a hot, charred glass tip. [25][26] The laser's irradiation is highly absorbed in the distal end of the tip and heats it to 500 °C to 900 °C.[25] Because the tip is so hot, it can cause hemostasis through cauterization and carbonization. [25][26] Diode lasers on soft tissue can cause extensive collateral thermal damage to surrounding tissue. [25][26][27]
As laser beam light is inherently coherent, some applications utilize the coherence of laser diodes. These include interferometric distance measurement, holography, coherent communications, and coherent control of chemical reactions.
Laser diodes, for their narrow spectral properties in the areas of range-finding, telecommunications, infrared countermeasures, spectroscopic sensing, generation of radio-frequency or terahertz waves, atomic clock state preparation, quantum key cryptography, frequency doubling and conversion, water purification (in the UV), and photodynamic therapy
Laser diodes, for their ability to generate ultra-short pulses of light by the technique known as mode-locking. Areas of use include clock distribution for high-performance integrated circuits, high-peak-power sources for laser-induced breakdown spectroscopy sensing, arbitrary waveform generation for radio-frequency waves, photonic sampling for analog-to-digital conversion, and optical code-division-multiple-access systems for secure communication. [9]

Recent developments:

HQP series includes two wavelengths of 1064nm and 532nm. The pulse duration can go down to 750ps (0.75ns). Its applications include laser ocean mapping and seafloor mapping (1-2).

• Compact design

• Lightweight and high reliability

• Perfect for laser ranging

 
In the near IR (Infrared) spectral region, the team reported a significant advancement with diode-pumped rare earth lasers based on the ions Nd, Tm, Ho, and Yb (3). The new class of Cr4+ transition metal ion lasers emit tunable transition radiation in a spectral region between 1.17µm and 1.63µm in the visible spectral region. 
 
Organic semiconductor lasers are a promising gain medium for novel laser devices due to their convenient processing technique [4]. 

• Easy spectral and chemical tuning

• Convenient processing technique

• Low refractive indices

• Mechanical flexibility

• Low cost

• Time-saving and environment-friendly manufacturing technologies.

Organic lasers have a great deal to offer under optical pumping [5].

The pumping is possible using cheap LEDs in organic lasers.  These developments continue to trigger fruitful relations between chemists and laser physicists and prove that there are still exciting new challenges in light-emitting molecular materials beyond organic light-emitting diode [7-8]

 

References

 

1 Sciencedirect.com

2 HQP Series of high peak power laser, en.real.light.com.

3 Opg.optica.org

4 Yi Jiang et al, Organic solid state lasers (OSSLs), Chemical Society Reviews, 16, 2022.

5 Sebastien Chenais &Sebastien Forget, Recent advances in Solid-State Organic Laser, Research gate.

6 T. A. Hassan et al., Engineering Physics, Aswathy pub,. 2010.

7  Peter Moulton, J. optical soc. Of America B3 (1), 125-133.

 8 Spence D E et al. Optics letters, 15 (1), 42-44.

9 Wikipedia

10 Chow, W. W.; Koch, S. W. (2011). Semiconductor-Laser fundamentals. Springer. ISBN 978-3540641667

11 Arrigoni, M. et al. (2009-09-28) "Optically Pumped Semiconductor Lasers: Green OPSLs poised to enter scientific pump-laser market", Laser Focus World

12 ^ "Optically Pumped Semiconductor Laser (OPSL)", Sam's Laser FAQs.
13^ Coherent white paper (2018-05). "Advantages of Optically Pumped Semiconductor Lasers – Invariant Beam Properties"

14 Steele, Robert V. (2005). "Diode-laser market grows at a slower rate". Laser Focus World. 41 (2). Archived from the original on 2006-04-08.
15^ Kincade, Kathy; Anderson, Stephen (2005). "Laser Marketplace 2005: Consumer applications boost laser sales 10%". Laser Focus World. 41 (1). Archived from the original on June 28, 2006.

16 Yeh, S; Jain, K; Andreana, S (2005). "Using a diode laser to uncover dental implants in second-stage surgery". General Dentistry. 53 (6): 414–7. PMID 16366049.
17^ Andreana, S (2005). "The use of diode lasers in periodontal therapy: literature review and suggested technique". Dentistry Today. 24 (11): 130, 132–5. PMID 16358809.
18 ^ Borzabadi-Farahani A (2017). "The Adjunctive Soft-Tissue Diode Laser in Orthodontics". Compend Contin Educ Dent. 37 (eBook 5): e18–e31. PMID 28509563.
19 ^ Borzabadi-Farahani, A. (2022). "A Scoping Review of the Efficacy of Diode Lasers Used for Minimally Invasive Exposure of Impacted Teeth or Teeth with Delayed Eruption". Photonics. 9 (4): 265. Bibcode:2022Photo...9..265B. doi:10.3390/photonics9040265.
20 ^ Deppe, Herbert; Horch, Hans-Henning (2007). "Laser applications in oral surgery and implant dentistry" (PDF). Lasers in Medical Science. 22 (4): 217–221. doi:10.1007/s10103-007-0440-3. PMID 17268764. S2CID 23606690.^{[permanent dead link]}
21 ^ Borzabadi-Farahani, A (2024). "Laser Use in Muco-Gingival Surgical Orthodontics". In Coluzzi, D.J.; Parker, S.P.A. (eds.). Lasers in Dentistry—Current Concepts. Textbooks in Contemporary Dentistry (2nd ed.). Springer, Cham. pp. 379–398. doi:10.1007/978-3-031-43338-2. ISBN 978-3-031-43338-2.
22 ^ Feuerstein, Paul (May 2011). "Cuts Like A Knife". Dental Economics. Retrieved 2016-04-12.
23 ^ Wright, V. Cecil; Fisher, John C. (1993-01-01). Laser Surgery in Gynecology: A Clinical Guide. Saunders. pp. 58–81. ISBN 9780721640075.
24 ^ Shapshay, S. M. (1987-06-16). Endoscopic Laser Surgery Handbook. CRC Press. pp. 1–130. ISBN 9780824777111.
25^ Jump up to:a b c d e Romanos, Georgios E. (2013-12-01). "Diode laser soft-tissue surgery: advancements aimed at consistent cutting, improved clinical outcomes". Compendium of Continuing Education in Dentistry. 34 (10): 752–7, quiz 758. PMID 24571504

26.^ Jump up to:a b c d Vitruk, PP (2015). "Oral Soft Tissue Laser Ablative and Coagulative Efficiencies Spectra". Implant Practice US. 7 (6): 19–27.

27 ^ Lingrong Jian; et al. (2016). "GaN-based green laser diodes". Journal of Semiconductors. 37 (11): 111001. Bibcode:2016JSemi..37k1001L. doi:10.1088/1674-4926/37/11/111001. S2CID 114572097.