Can various diode lasers of equal power be mixed to produce sum/differce freq laser?

[frank_1257]

I am curious if I could take lets say an 500mw 382nm laser diode and a lets say 910nm diode of the same power output density for a astigmatically corrected collimated beam/spot size and mix the 2 lasers and produce the difference frequency of 528nm or some other similar combination of various high power laser diodes at 382nm-920nm, I am used to doing this in audio work microwave uhf vhf. Can this be done by mixing of two lasers to produce the sum or difference frequencies???

[thesk8nmidget]

i have never thought about that before! good question!

im looking forward to the answer.

but here a guess... i dont think two lasers in the non visible spectrum will mix to make something visible. but i'll let the experts do the talking lol

honestly i hope im wrong!

[motorider826]

pretty sure no, since lasers use coherent light.
instead of the colors combining to form a new one, they instead lay on top of each other.
the only reason why scanners work is that your eye sees the different colors and combines them in your mind. i think. i may be way off tho.
EDIT- So non visible wavelengths combined do not make visible is what im saying, as far as i understand

[frank_1257]

pretty sure no, since lasers use coherent light.
instead of the colors combining to form a new one, they instead lay on top of each other.
the only reason why scanners work is that your eye sees the different colors and combines them in your mind. i think. i may be way off though.
EDIT- So non visible wavelengths combined do not make visible is what im saying, as far as i understand

This is certainly a question for the physicist amongst us.
I know that audio frequencies can be beat together as I used to tune black boxes to hack the usa phone system in 1985 by disabling each of the paired tones for the 3 columns 4 rows and the watts line clear pair 2600hz and 3400hz all one at a time I had one tuned box and as the other approached its frequency the beat offset would approach zero and cross and then increase in frequency again.

I know that in vhf uhf and microwave's to 10's of GHZ that electromagnetic waves can be beat to together to produce the difference or sum frequencies.

If this same phenomenon occurs in light waves where photons may behave differently then hypothetically one could take two light waves that are ideally pure coherent monochromatic single frequency lasers and could in some appropriate mixing medium create two additional waves from the mixing of the first two waves lets say for example a 382nm source and a 910nm diode to produce a desired 528nm as well as an undesired summation wave at 1292nm.

You run into one issue if this would work I have learned that shorter wavelength lasers have a different set of emission angles ( fast and slow axis ) as the frequency changes so the size of the laser diode emission area and this would change the effective light density leaving the diode even if both were matched for power I assume you would need to create similar sized and powered laser collimated or focused spots on this mixer. This might require diodes that are significantly different in their actual wattage to get two matched energy coherent light sources all entering a crystal to mix them and produce the desired difference light wave. I am seeing lasers that utilize multiple YAG line production in producing 488? yellow from the interfering 1319nm and 1064nm lines. There must be some reason that people are not producing a desired laser light from mixing of two other coherent lasers, I am guessing it can be done though.

[James Lehman]

I think audio is different than light because audio is a transverse wave of energy that moves through a medium. Therefore, a phenomena called super position comes into play. Sine waves of two different frequencies add together in a very special way in a transfer medium.


Light is a particle that moves like a wave, but it can travel through a vacuum.

Maybe this is just pure BS.

James.

[frank_1257]

Sum frequency generation

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Sum-frequency generation (SFG) is an example of a second order non-linear optical process. This phenomenon is based on the annihilation of two input photons at frequencies ω1 and ω2 while, simultaneously, one photon at frequency ω3 is generated. In order that the sum-frequency generation takes place it is necessary that the following two conditions happen:

1. Energy conservation: Ok so the energy of the 3rd produced sum wave has to equal the combined energy of the first 2 primary waves.

2. Momentum conservation:

Oh god NO!! not That, In my search of further comprehension I googled my self into a I came I saw I clicked : Physics Help and Math Help Oh God NO! Physics student Forum Oh Well!
http://www.physicsforums.com/archive.../t-102574.html


The first condition shows the relationship of the frequencies between the input and output photons. It is possible to notice that the sum-frequency generation is a generalization of second harmonic generation. In the latter, ω1 = ω2, both of which can be provided by a single light source. The second condition implicates the k-vector conservation, in fact Δk = k3 − k2 − k1 = 0; this condition is called phase matching condition.


In a common SFG application, light from a tunable infrared laser is combined with light from a fixed infrared frequency in order to produce visible light through wave mixing process.

In my desired attempt I would be producing a visible coherent wave from a UV 382nm source and a IR 910nm source ? Could this work?

Frequency mixing processes

• Second harmonic generation (SHG), or frequency doubling, generation of light with a doubled frequency (half the wavelength);
• Sum frequency generation (SFG), generation of light with a frequency that is the sum of two other frequencies (SHG is a special case of this);
• Third harmonic generation (THG), generation of light with a tripled frequency (one-third the wavelength) (usually done in two steps: SHG followed by SFG of original and frequency-doubled waves);
• Difference frequency generation (DFG), generation of light with a frequency that is the difference between two other frequencies;
• Parametric amplification, amplification of a signal input in the presence of a higher-frequency pump wave, at the same time generating an idler wave (can be considered as DFG);
• Parametric oscillation, generation of a signal and idler wave using a parametric amplifier in a resonator (with no signal input);
• Parametric generation, like parametric oscillation but without a resonator, using a very high gain instead;
• Spontaneous parametric down conversion (SPDC), the amplification of the vacuum fluctuations in the low gain regime;
• Optical rectification, generation of quasi-static electric fields.
• Four-wave mixing (FWM), can also arise from other nonlinearities.

[edit] Other nonlinear processes

• Optical Kerr effect, intensity dependent refractive index;
  
  • Self-focusing;
  • Kerr-lens modelocking (KLM).
  • Self-phase modulation (SPM), a χ(3) effect.
  • Optical solitons.
  • Cross-phase modulation (XPM);
  • Four-wave mixing (FWM), can also arise from other nonlinearities.
  • Cross-polarized wave generation (XPW), a χ(3) effect in which a wave with polarization vector perpendicular to the input one is generated.
  • Raman amplification,
  • Optical phase conjugation.
  
  
• Brillouin scattering, interaction of photons with acoustic phonons;
  
  • Optical phase conjugation.
  
  
• Two-photon absorption, simultaneous absorption of two photons, transferring the energy to a single electron;
• Multiple photoionisation, near-simultaneous removal of many bound electrons by one photon.
• Chaos in Optical Systems

[edit] Related processes

In these processes, the medium has a linear response to the light, but the properties of the medium are affected by other causes:

• Pockels effect, the refractive index is affected by a static electric field; used in electro-optic modulators;
• Acousto-optics, the refractive index is affected by acoustic waves (ultrasound); used in acousto-optic modulators.
• Raman scattering, interaction of photons with optical phonons;

[edit] Frequency-mixing processes

One of the most commonly-used frequency-mixing processes is frequency doubling or second-harmonic generation. With this technique, the 1064-nm output from Nd:YAG lasers or the 800-nm output from Ti:sapphire lasers can be converted to visible light, with wavelengths of 532 nm (green) or 400 nm (violet), respectively.
Practically, frequency-doubling is carried out by placing a special crystal in a laser beam under a well-chosen angle. Commonly-used crystals are BBO (β-barium borate), KDP (potassium dihydrogen phosphate), KTP (potassium titanyl phosphate), and lithium niobate. These crystals have the necessary properties of being strongly birefringent (necessary to obtain phase matching, see below), having a specific crystal symmetry and of course being transparent for and resistant against the high-intensity laser light. However, organic polymeric materials are set to take over from crystals as they are cheaper to make, have lower drive voltages and superior performance.

[edit] Theory

A number of nonlinear optical phenomena can be described as frequency-mixing processes. If the induced dipole moments of the material respond instantaneously to an applied electric field, the dielectric polarization (dipole moment per unit volume) P(t) at time t in a medium can be written as a power series in the electrical field:
. Here, the coefficients χ(n) are the n-th order susceptibilities of the medium. For any three-wave mixing process, the second-order term is crucial; it is only nonzero in media that have no inversion symmetry. If we write
, where c.c. denotes the complex conjugate (E1 and E2 being the incident beams of interest), the second-order term in the above expansion will read
, where the summation is over
. The six combinations (nx,mx) correspond, respectively, to the second harmonic of E1, the second harmonic of E2, the optically rectified signals of E1 and E2, the difference frequency, and the sum frequency. A medium that is thus pumped by the fields E1 and E2 will radiate a field E3 with an angular frequency ω3 = m1ω1 + m2ω2.
Note: in this description, χ(2) is a scalar. In reality, χ(2) is a tensor whose components depend on the combination of frequencies.
Parametric generation and amplification is a variation of difference frequency generation, where the lower-frequency one of the two generating fields is much weaker (parametric amplification) or completely absent (parametric generation). In the latter case, the fundamental quantum-mechanical uncertainty in the electric field initiates the process.

[edit] Phase matching

The above ignores the position dependence of the electrical fields. In a typical situation, the electrical fields are traveling waves described by
, at position , with the wave vector , where c is the velocity of light and n(ωj) the index of refraction of the medium at angular frequency ωj. Thus, the second-order polarization angular frequency ω3 is
. At each position , the oscillating second-order polarization radiates at angular frequency ω3 and a corresponding wave vector . Constructive interference, and therefore a high intensity ω3 field, will occur only if
. The above equation is known as the phase matching condition. Typically, three-wave mixing is done in a birefringent crystalline material (I.e., the refractive index depends on the polarization and direction of the light that passes through.), where the polarizations of the fields and the orientation of the crystal are chosen such that the phase-matching condition is fulfilled. This phase matching technique is called angle tuning. Typically a crystal has three axes, one of which has a different refractive index than the other ones. This axis is called the extraordinary (e) axis, while the other two are ordinary axes (o). There are several schemes of choosing the polarizations. If the signal and idler have the same polarization, it is called "Type-I phase-matching", and if their polarizations are perpendicular, it is called "Type-II phase-matching". However, other conventions exist that specify further which frequency has what polarization relative to the crystal axis. These types are listed below, with the convention that the signal wavelength is shorter than the idler wavelength.
Phase-matching types ()
Polarizations Scheme Pump Signal Idler
e o o Type I e o e Type II (or IIA) e e o Type III (or IIB) e e e Type IV o o o Type V o o e Type VI (or IIB or IIIA) o e o Type VII (or IIA or IIIB) o e e Type VIII (or I) Most common nonlinear crystals are negative unaxial, which means that the e axis has a smaller refractive index than the o axes. In those crystals, type I and II phasematching are usually the most suitable schemes. In positive uniaxial crystals, types VII and VIII are more suitable. Types II and III are essentially equivalent, except that the names of signal and idler are swapped when the signal has a longer wavelength than the idler. For this reason, they are sometimes called IIA and IIB. The type numbers V–VIII are less common than I and II and variants.
One undesirable effect of angle tuning is that the optical frequencies involved do not propagate collinearly with each other. This is due to the fact that the extraordinary wave propagating through a birefringent crystal possesses a Poynting vector that is not parallel with the propagation vector. This would lead to beam walkoff which limits the nonlinear optical conversion efficiency. Two other methods of phase matching avoids beam walkoff by forcing all frequencies to propagate at a 90 degree angle with respect to the optical axis of the crystal. These methods are called temperature tuning and quasi-phase-matching.
Temperature tuning is where the pump (laser) frequency polarization is orthogonal to the signal and idler frequency polarization. The birefringence in some crystals, in particular Lithium Niobate is highly temperature dependent. The crystal is controlled at a certain temperature to achieve phase matching conditions.
The other method quasi-phase matching. In this method the frequencies involved are not constantly locked in phase with each other, instead the crystal axis is flipped at a regular interval Λ, typically 15 micrometres in length. Hence, these crystals are called periodically-poled. This results in the polarization response of the crystal to be shifted back in phase with the pump beam by reversing the nonlinear susceptibility. This allows net positive energy flow from the pump into the signal and idler frequencies. In this case, the crystal itself provides the additional wavevector k=2π/λ (and hence momentum) to satisfy the phase matching condition. Quasi-phase matching can be expanded to chirped gratings to get more bandwidth and to shape an SHG pulse like it is done in a dazzler. SHG of a pump and Self-phase modulation (emulated by second order processes) of the signal and an optical parametric amplifier can be integrated monolithically.

[edit] Higher-order frequency mixing





The above holds for χ(2) processes. It can be extended for processes where χ(3) is nonzero, something that is generally true in any medium without any symmetry restrictions. Third-harmonic generation is a χ(3) process, although in laser applications, it is usually implemented as a two-stage process: first the fundamental laser frequency is doubled and then the doubled and the fundamental frequencies are added in a sum-frequency process. The Kerr effect can be described as a χ(3) as well.
At high intensities the Taylor series, which led the domination of the lower orders, does not converge anymore and instead a time based model is used. When a noble gas atom is hit by an intense laser pulse, which has an electric field strength comparable to the Coulomb field of the atom, the outermost electron may be ionized from the atom. Once freed, the electron can be accelerated by the electric field of the light, first moving away from the ion, then back toward it as the field changes direction. The electron may then recombine with the ion, releasing its energy in the form of a photon. The light is emitted at every peak of the laser light field which is intense enough, producing a series of attosecond light flashes. The photon energies generated by this process can extend past the 800th harmonic order up to 1300 eV. This is called high-order harmonic generation. The laser must be linearly polarized, so that the electron returns to the vicinity of the parent ion. High-order harmonic generation has been observed in noble gas jets, cells, and gas-filled capillary waveguides.

[edit] Optical phase conjugation


Comparison of a phase conjugate mirror with a conventional mirror. With the phase conjugate mirror the image is not deformed when passing through an aberrating element twice.

[frank_1257]

It is possible, using nonlinear optical processes, to exactly reverse the propagation direction and phase variation of a beam of light. The reversed beam is called a conjugate beam, and thus the technique is known as optical phase conjugation (also called time reversal, wavefront reversal and retroreflection).
One can interpret this nonlinear optical interaction as being analogous to a real-time holographic process. In this case, the interacting beams simultaneously interact in a nonlinear optical material to form a dynamic hologram (two of the three input beams), or real-time diffraction pattern, in the material. The third incident beam diffracts off this dynamic hologram, and, in the process, reads out the phase-conjugate wave. In effect, all three incident beams interact (essentially) simultaneously to form several real-time holograms, resulting in a set of diffracted output waves that phase up as the "time-reversed" beam. In the language of nonlinear optics, the interacting beams result in a nonlinear polarization within the material, which coherently radiates to form the phase-conjugate wave.

The most common way of producing optical phase conjugation is to use a four-wave mixing technique, though it is also possible to use processes such as stimulated Brillouin scattering. A device producing the phase conjugation effect is known as a phase conjugate mirror (PCM).
For the four-wave mixing technique, we can describe four beams (j = 1,2,3,4) with electric fields:
where Ej are the electric field amplitudes. Ξ1 and Ξ2 are known as the two pump waves, with Ξ3 being the signal wave, and Ξ4 being the generated conjugate wave.
If the pump waves and the signal wave are superimposed in a medium with a non-zero χ(3), this produces a nonlinear polarization field:
PNL = ε0χ(3)(Ξ1 + Ξ2 + Ξ3)3 resulting in generation of waves with frequencies given by ω = ±ω1 ±ω2 ±ω3 in addition to third harmonic generation waves with ω = 3ω1, 3ω2, 3ω3.
As above, the phase-matching condition determines which of these waves is the dominant. By choosing conditions such that ω = ω1 + ω2 - ω3 and k = k1 + k2 - k3, this gives a polarization field:
. This is the generating field for the phase conjugate beam, Ξ4. Its direction is given by k4 = k1 + k2 - k3, and so if the two pump beams are counterpropagating (k1 = -k2), then the conjugate and signal beams propagate in opposite directions (k4 = -k3). This results in the retroreflecting property of the effect.
Further, it can be shown for a medium with refractive index n and a beam interaction length l, the electric field amplitude of the conjugate beam is approximated by
(where c is the speed of light). If the pump beams E1 and E2 are plane (counterpropagating) waves, then:
; that is, the generated beam amplitude is the complex conjugate of the signal beam amplitude. Since the imaginary part of the amplitude contains the phase of the beam, this results in the reversal of phase property of the effect.
Note that the constant of proportionality between the signal and conjugate beams can be greater than 1. This is effectively a mirror with a reflection coefficient greater than 100%, producing an amplified reflection. The power for this comes from the two pump beams, which are depleted by the process.
The frequency of the conjugate wave can be different from that of the signal wave. If the pump waves are of frequency ω1 = ω2 = ω, and the signal wave higher in frequency such that ω3 = ω + Δω, then the conjugate wave is of frequency ω4 = ω - Δω. This is known as frequency flipping.

[frank_1257]

Must read on waiting on that know it all physicist in our midst um speak up now, ah please!

[frank_1257]

Motonobu Kourogi3, 4 , Kazuhiro Imai3, Bambang Widiyatmoko3 and Motoichi Ohtsu3, 4
(3) Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan (4) Kanagawa Academy of Science and Technology, KSP East Building, Room 408,2-1,Sakado 3-chome,Takatsu-ku, Kawasaki 213-0012, Japan Abstract
We present a method to expand optical frequency combs (OFC) using self-phase modulation in an optical fiber. The initial OFC is generated using a resonant electro-optic modulator and exhibits a span of less than 10 THz. The span of a broadened OFC can reach up to 50 THz in the 1.5 µm wavelength domain. Second-Harmonic Generation (SHG) of this OFC has also been demonstrated. The span of a second-harmonic comb can also reach up to 50 THz but in the 0.8 µm region. We also demonstrate an innovative method to make frequency-difference measurements between two laser signals when the difference frequency between the lasers is larger than the span of the OFC.

[frank_1257]

OK I AM ADMITTEDLY LOST THiS HAS GONE OVER MY HEAD!



Home Research Faculty Graduate & Postdoctoral Studies


A Brief History of Optical Frequency Combs


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PHASE 1: 1960–1990: The divergence of laser stabilization and ultrafast laser development

• 1960:
  
  Theodore Maiman demonstrates the first working laser, solid-state ruby laser, launching thirty years of research into ultrafast laser development.
  
  Ali Javan demonstrates the first gas laser with William Bennett, initiating three decades of research in laser stabilization.
• 1972–1983:
  
  Stabilized lasers allow new, definitive measurement of the speed of light in 1972, leading to the 1983 redefinition of the meter based on the speed of light.
• Mid 1970s:
  
  Introduction of synchronously pumped mode-locked dye lasers.
• 1977–1978:
  
  Theodore Hansch’s team at Stanford uses an early mode-locked picosecond dye laser to perform a landmark experiment showing that stable pulse trains could produce stable combs of frequencies. Veniamin Chebotayev independently predicts that stable, repetitive laser pulse trains could be used for optical frequency metrology.
• 1986:
  
  Invention of the titanium-doped sapphire laser.

PHASE 2: 1990–2001: The convergence of laser stabilization and ultrafast laser development

• 1990–1991:
  
  Wilson Sibbett invents Kerr lens mode-locking.
• 1991:
  
  First commercial Ti:sapphire laser is introduced.
• 1993:
  
  M. Kourogi and colleagues demonstrate intracavity modulator-based spectral comb generators.
• 1991–1995:
  
  Ted Hanch’s group in Garching begins to investigate the possibility of laser pulses short enough to produce frequencies spanning from the radio up into the visible domain.
  
  Pulse width in Ti:sapphire lasers is reduced from 100+ fs down to 10 fs. Repetition rate in mode-locked Ti:sapphire lasers reaches gigahertz range.
• 1997:
  
  Theodore Hansch proposes an octave-spanning self-referenced universal optical frequency comb synthesizer.
• 1998:
  
  Test of a Kerr lens mode-locked Ti’sapphire laser in Garching proves the viability of femtosecond laser frequency comb synthesizers.
• 1999:
  
  Jinendra Ranka, Robert Windeler, and Andrew Stentz demonstrated the use of internally structured fiber to produce of white light composed of a supercontinuum of infrared, red, green and blue light.
  
  John Hall and Steve Cundiff verify the supercontinuum idea at JILA.
• 2000:
  
  John Hall, Steve Cundiff and Jun Ye prove that the output of the "magic" fiber is a spectral comb of coherent frequencies. The JILA team measures and controls the carrier-envelope offset frequency with a self-referenced comb and determines the frequency offset of a frequency comb. The JILA team learns to control the pulse-to-pulse carrier-envelope offset phase.
  
  The JILA and Garching teams report the first absolute optical frequency measurement with a frequency comb in a single step.
  
  The Garching team establishes the viability of optical frequency metrology with femtosecond laser frequency combs. The team also reports the first self-contained radio-frequency to optical frequency comparison.
• 2001:
  
  The age of optical atomic clock development begins when Jim Bergquist at NIST incorporates a frequency comb into the world’s first optical atomic clock based on a single ion of mercury. John Hall and Jun Ye demonstrate the first molecular optical clock at JILA.
• 2001–2005:
  
  Leo Hollberg (NIST) begins work on an optical atomic clock based on neutral calcium atoms. Jun Ye (JILA) initiates development of a neutral strontium lattice optical atomic clock.
  
  The Jun Ye group at JILA generates the world’s first frequency comb in the extreme ultraviolet (XUV).

For more information on recent research at JILA on optical frequency comb applications, please see:

• Light Control
• There’s Strontium in the Clock
• Time Traveling
• Molecular Fingerprinting
• Partnership in Time
• The South Broadway Shootout
• Clock Talk coming soon....

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