Today's high-power electronic and optical devices require better heat transfer to the heat sink. Gold-tin (AuSn) solder is the answer to such packaging challenges. This is because AuSn solder has superior high-temperature performance, excellent electrical and thermal conductivity, high mechanical strength, and fluxless soldering [1].
Today I read from nLight's website [2] that they use AuSn soldering of high-power diode emitters onto the coefficient-of-thermal-expansion (CTE) matched heatsinks (such as BeO). Compared to the traditional indium soldering ("soft soldering") onto copper heatsink, the AuSn soldering ("hard soldering") has significantly increased reliability.
References:
[1] http://www.flipchips.com/tutorial46.html
[2] http://nlight.net/nlight-files/file/technical_papers/LEOS2008-AnnualMeeting_Leisher.pdf
Sunday, January 18, 2009
Thursday, January 15, 2009
About VCSEL
VCSEL: Vertical Cavity Surface Emitting Laser.
VCSEL's laser resonator consists of two distributed Bragg reflector (DBR) mirrors with high reflectance >99%. This high reflectance is required to compensate for the short axial length of the gain region.
Advantages (compared to edge-emitters, of course):
1. Laser cavity is short (1~1.5 λ), so only one longitudinal mode can oscillate. (The longitudinal mode spacing is λ/2).
2. The high reflective resonator mirrors results in low threshold current so VCSEL has lower power consumption. (However lower output optical power.)
3. λ vs. T (<0.1 nm/K) is ~5 times smaller than edge emitters (0.2~0.3 nm/K).
4. Easier thermal dissipation and high T operation.
5. Circular output beam. This brings easier beam shaping, easier fiber-coupling, etc.
6. High reliability. VCSEL is not subject to catastrophic optical damage (COD).
7. When put in external-cavity, EC-VCSEL has no mode-hops during tuning or modulation because of the large mode-spacing.
Disadvantages:
Need to do more search on this.
The brightness of the high-power VCSEL is still lower than edge-emitters (why?)
References:
[1] Princeton Optronix website.
[2] http://en.wikipedia.org/wiki/VCSEL
VCSEL's laser resonator consists of two distributed Bragg reflector (DBR) mirrors with high reflectance >99%. This high reflectance is required to compensate for the short axial length of the gain region.
Advantages (compared to edge-emitters, of course):
1. Laser cavity is short (1~1.5 λ), so only one longitudinal mode can oscillate. (The longitudinal mode spacing is λ/2).
2. The high reflective resonator mirrors results in low threshold current so VCSEL has lower power consumption. (However lower output optical power.)
3. λ vs. T (<0.1 nm/K) is ~5 times smaller than edge emitters (0.2~0.3 nm/K).
4. Easier thermal dissipation and high T operation.
5. Circular output beam. This brings easier beam shaping, easier fiber-coupling, etc.
6. High reliability. VCSEL is not subject to catastrophic optical damage (COD).
7. When put in external-cavity, EC-VCSEL has no mode-hops during tuning or modulation because of the large mode-spacing.
Disadvantages:
Need to do more search on this.
The brightness of the high-power VCSEL is still lower than edge-emitters (why?)
References:
[1] Princeton Optronix website.
[2] http://en.wikipedia.org/wiki/VCSEL
Wednesday, January 14, 2009
About Fiber
I need to go over some basic knowledge on fibers.
1. V number.
The number of modes in a step index circular waveguide is determined by the V number.
For single-mode fiber, V < 2.405. For multi-mode fiber the number of modes allowed in the fiber is approximately V2/2.
2. Numerical aperture.
The sine of the half acceptance angle is NA.
NA = (ncore2 - nclad2)1/2.
For reference,
f# = 1/(2 NA).
Note that it is not necessarily true that a fiber's output beam's divergence angle will equal to its acceptance angle. Instead, the fiber tends to preserve the angle of incidence during propagation of the light, causing it to exit the fiber at the same angle it entered.
References:
1. http://www.fiberoptix.com/technical/numerical-aperature.html
2. Wilson and Hawkes, Optoelectronics, 3rd ed., Prentice Hall Europe 1998.
1. V number.
The number of modes in a step index circular waveguide is determined by the V number.
| V = | 2πaNA λ0 | (1) |
For single-mode fiber, V < 2.405. For multi-mode fiber the number of modes allowed in the fiber is approximately V2/2.
2. Numerical aperture.
The sine of the half acceptance angle is NA.
NA = (ncore2 - nclad2)1/2.
For reference,
f# = 1/(2 NA).
Note that it is not necessarily true that a fiber's output beam's divergence angle will equal to its acceptance angle. Instead, the fiber tends to preserve the angle of incidence during propagation of the light, causing it to exit the fiber at the same angle it entered.
References:
1. http://www.fiberoptix.com/technical/numerical-aperature.html
2. Wilson and Hawkes, Optoelectronics, 3rd ed., Prentice Hall Europe 1998.
Monday, January 12, 2009
Second Harmonic Generation
A few issues here:
1. How to explain or understand phase-matching easily.
Well, there is dispersion in all materials. For fundamental and 2nd harmonic waves, different wavelength causes different refractive index and then causes different traveling velocity. Consequently, along the propagation direction, the previously-generated 2nd-harmonic-wave and the newly-generated 2nd-harmonic-wave will have different phase. Thus destructive interference will happen and this is phase-mismatching. To fulfill phase-matching, we use birefringence property of some crystals. At certain direction, the cross-polarized fundamental and 2nd-harmonic waves can have the same traveling speed.
2. Type-I and type-II phase-matching.
Type-I: the signal and idler beams have the same polarization. In SHG, signal and idler are equal, which are both the fundamental beam. So in SHG, type-I phase-matching is for polarized fundamental laser.
Type-II: the signal and idler beams have perpendicular polarization. In SHG, type-II phase-matching is for unpolarized fundamental laser.
3. Impedance matched cavity.
For cavity-enhanced SHG, we want the transmission of the input mirror at the pump wavelength equals all intracavity pump losses, including the loss from nonlinear frequency conversion. This way the initial pump reflection and the cavity leakage field will cancel (they always have opposite phase), resulting in zero total pump reflection. Thus all pump light will be coupled into cavity and maximize the conversion efficiency.
4. Major drawback of the intracavity SHG.
It is called "green noise", the intensity instability caused by interaction of multiple longitudinal modes of the fundamental laser.
1. How to explain or understand phase-matching easily.
Well, there is dispersion in all materials. For fundamental and 2nd harmonic waves, different wavelength causes different refractive index and then causes different traveling velocity. Consequently, along the propagation direction, the previously-generated 2nd-harmonic-wave and the newly-generated 2nd-harmonic-wave will have different phase. Thus destructive interference will happen and this is phase-mismatching. To fulfill phase-matching, we use birefringence property of some crystals. At certain direction, the cross-polarized fundamental and 2nd-harmonic waves can have the same traveling speed.
2. Type-I and type-II phase-matching.
Type-I: the signal and idler beams have the same polarization. In SHG, signal and idler are equal, which are both the fundamental beam. So in SHG, type-I phase-matching is for polarized fundamental laser.
Type-II: the signal and idler beams have perpendicular polarization. In SHG, type-II phase-matching is for unpolarized fundamental laser.
3. Impedance matched cavity.
For cavity-enhanced SHG, we want the transmission of the input mirror at the pump wavelength equals all intracavity pump losses, including the loss from nonlinear frequency conversion. This way the initial pump reflection and the cavity leakage field will cancel (they always have opposite phase), resulting in zero total pump reflection. Thus all pump light will be coupled into cavity and maximize the conversion efficiency.
4. Major drawback of the intracavity SHG.
It is called "green noise", the intensity instability caused by interaction of multiple longitudinal modes of the fundamental laser.
Friday, January 9, 2009
About Fiber Laser
Fiber lasers generally use fiber Bragg grating to form the laser cavity.
Advantages:
1. Fiber lasers can produce high power light at excellence beam quality.
2. Fiber lasers use rare-earth doped glass fibers, which has larger gain bandwidth compared with the rare-earth doped bulk crystals. Fiber lasers thus can have broad tuning range and can generate short pulses via mode-locking.
3. Fiber lasers have large surface-to-volume ratio and are easier for thermal management.
4. Fiber lasers are very mechanically stable against vibrations.
Difficulties:
1. Efficiently coupling pump diodes into the fiber has tight alignment tolerance.
2. Fibers have birefringence and polarization control is difficult.
Fiber lasers use double-clad fiber. The gain medium is in the center core, where the lasing mode propagates; whereas the inner cladding layer contains the pump light.
To generate single-mode laser beam at high power, and to decrease nonlinearity, etc, "large mode area single-mode fiber" is now used for fiber laser [2]. Since it has large mode area, to ensure single-mode propagation, the fiber must have a low NA (keep V-number low). For example, a Yb-doped fiber laser has a 20um core (NA=0.06) and a 400um clad (NA=0.46).
Fiber lasers often are broad-band lasing to eliminate Brillouin scattering.
References:
[1] http://en.wikipedia.org/wiki/Fiber_laser
[2] http://www.nufern.com/whitepaper_detail.php/30
Advantages:
1. Fiber lasers can produce high power light at excellence beam quality.
2. Fiber lasers use rare-earth doped glass fibers, which has larger gain bandwidth compared with the rare-earth doped bulk crystals. Fiber lasers thus can have broad tuning range and can generate short pulses via mode-locking.
3. Fiber lasers have large surface-to-volume ratio and are easier for thermal management.
4. Fiber lasers are very mechanically stable against vibrations.
Difficulties:
1. Efficiently coupling pump diodes into the fiber has tight alignment tolerance.
2. Fibers have birefringence and polarization control is difficult.
Fiber lasers use double-clad fiber. The gain medium is in the center core, where the lasing mode propagates; whereas the inner cladding layer contains the pump light.
To generate single-mode laser beam at high power, and to decrease nonlinearity, etc, "large mode area single-mode fiber" is now used for fiber laser [2]. Since it has large mode area, to ensure single-mode propagation, the fiber must have a low NA (keep V-number low). For example, a Yb-doped fiber laser has a 20um core (NA=0.06) and a 400um clad (NA=0.46).
Fiber lasers often are broad-band lasing to eliminate Brillouin scattering.
References:
[1] http://en.wikipedia.org/wiki/Fiber_laser
[2] http://www.nufern.com/whitepaper_detail.php/30
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