Guidelines on picking the suitable crystal
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| Factors to consider for Laser Crystal Selection |
| Transmittance of Crystal | Environment Condition | Birefringent Walk off | Damage Threshold | Conversion Efficiency | Spectral Acceptance | Pulse Width |
| Effective linear/non-linear coefficient | Acceptance Angel | Repetition Rate | Group Velocity Mismatch | Phase-Matching Type and Angle | Temperature Acceptance & Moisture | Cutting angle & Crystal size |
Considerations for Picking the right laser crystal:
Transmittance of Crystal:
The distribution of the diffractive elements in the material can affect light transmission in inhomogeneous photonic mediums, which is governed by a linear law called Shannon index. One factor to consider when choosing a crystal is the homogeneity of the structure, since the more homogeneous the crystal is, the more light will be transmitted.
Damage Threshold:
As a guide, it is recommended to choose a crystal that has a much higher damage threshold than the laser systems you are using. Choosing a crystal that has damage threshold a little higher than the laser system is not recommended even though it is within the range is because the damage threshold at the surface can be much lower than the bulk.
Effective linear/non-linear coefficient
The magnitude of the effective nonlinear coefficient deff, depends on the nonlinear tensor components and on the phase-matching configuration. It is important especially if the achievable optical intensities are low. The maximum non-linear coefficient might also not be at the optimum angle for perfect phase matching for some crystals , the crystal you pick would have to be customized for perfect phase matching.
The linear coefficient is a component of unit volume of the crystals and is correlated to the coefficient of expansion, this relates to heating at high optical power levels, and would disturb phase matching and thermal lensing can occur.
Group Velocity Mismatch:
The group velocity mismatch of a crystals limits the width of the phase-matching bandwidth. The group velocity mismatch can have great impacts for frequency conversion for e.g 10-ps pulses, and even stronger effects for femtosecond pulses, we generally use short crystals for converting short pulses since they need higher optical intensity for high conversion efficiency. To find the suitable crystal so as to achieve efficiency frequency conversion without significant pulse broadening, the thickness of the crystal should be less than pulse width/group velocity mismatching.
Cutting angle & Crystal Size: (NLO)
For non-linear crystals, there are three dimensions, Width(W), Height(H), and Length(L)mm3. The optimal crystal height should be slighter larger than the incoming laser beam diameter. The length of the crystal also affects the conversion efficiency.
As the laser beam diameter on the NLO crystal and the tuneable wavelength range is affected by the width, we use W=2H for wide wavelength tuning. In the case of frequency-doubling a crystal from q=30° to 60°, we get W= H + tg((60°-30°)/2) x L.
As for the length of the crystal, it must be decided on a case to case basis. For example, OPO and OPA processes would require longer crystals while ultrashort pulse lasers require very thin crystals.
Conversion Efficiency:
Conversion Efficiency is a function of the input wavelength and the length of the crystal used. To achieve the conversion efficiency you need, you need to vary the length of the crystal or input wavelength.
Environment Condition:
The environment where the crystal is kept and used matters, as some hygroscopic crystal materials such as KDP and BBO deteriorate when they are not kept in adequately dry air. Keep in mind whether your working environment is suited to the crystal you are thinking of purchasing.
Phase Matching Type & Angle
There are two types of Phase Matching, Type 1 and Type 2. Type 1 phase matching is sum frequency generation where two interacting beams have same polarization and perpendicular to that of the sum frequency wave. Type 2 phase matching is where the two interacting beams have different polarization directions and the birefringence is relative strong or the phase velocity mismatch is small. To achieve phase matching, you can either use angle tuning which is dependent on orientation of the crystal you are choosing or by temperature tuning the crystal.
Thus, to select the right crystal, the orientations of the crystal and thermal properties of the crystal need to be considered.
Acceptance Angle: (NLO)
The acceptance angle (Dq) is when a laser light propagates in the direction with the acceptance angle to phase matching direction. Dq is the full angle at half maximum (FAHM). Thus, if the crystal has a acceptance angle of 1 mradcm-1, a laser beam with divergence of 5mradcm-1 would have wasted power input. Thus, the acceptance angle of the crystal you are looking at should have an acceptance angle that is greater or equal to the power input/laser system.
Spectral Acceptance:
The efficiency of second‐harmonic generation (SHG) in a crystal is affected by its laser spectral width. The chromatic dispersion and birefringence properties also determines the possibilities for phase matching, phase-matching bandwidth and angular acceptance.
Birefringent Walk-off:(NLO)
The birefringence of NLO crystals will create a Poynting vector walk-off. When the beam size of input laser is small, the output beam and input beam will have a difference of walk-off angle (r) in the crystal, causing low conversion efficiency. Choosing a crystal depends on whether the walk-off will limit high conversion efficiency.
Repetition Rate: (NLO)
Repetition rate is often used to achieve high average power. SHG has instantaneous response time up till fs pulses, making it suitable for high repetition rate without deleterious effects and improves SHG average power. A crystal that can suit the repetition rate to get ideal SHG is desirable.
Pulse Width: (NLO)
Pulse width is the time difference between the leading and trailing edges of a single pulse of input source. In NLO, the shorter the pulse width of the crystal, the broader the spectrum we will get. To get the required information that you need, be sure to select a crystal that has the minimal pulse width that you require.
Contact Us | Product Catalogue | Technical Specialist: Majid (MathewPF@Simtrum.com)