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686

Periodically Poled Lithium Niobate (PPLN) - Tutorial

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Optics

Periodically poled lithium niobate (PPLN) isa highly efficient medium for nonlinearwavelength conversion processes. PPLN isused for frequency doubling, differencefrequency generation, sum frequencygeneration, optical parametric oscillation, andother nonlinear processes.

Principles Second order nonlinear processes involve themixing of three electromagnetic waves, wherethe magnitude of the nonlinear response ofthe crystal is characterized by the χ(2)

coefficient. Frequency doubling (SecondHarmonic Generation, SHG) is the mostcommon application that utilizes the χ(2)

properties of a nonlinear crystal. In SHG, twoinput photons with the same wavelength λ1

are combined through a nonlinear process togenerate a third photon at λ1/2. Similar toSHG, Sum Frequency Generation (SHG)combines two input photons at λ1 and λ2 togenerate an output photon at λgenerated with1/λgenerated = 1/λ1 + 1/λ2. Alternatively, inDifference Frequency Generation (DFG) thetwo input photons at λ1 and λ2 are combinedto generate an output photon at λgenerated with1/λgenerated = 1/λ1 - 1/λ2. Nonlinear processeswhere the frequency of the generated photonis not determined by the frequency of theinput photon are termed parametric processes.In a parametric process, a single input photonis split into two generated photons where theonly restriction on the combination offrequencies of the generated photons is that itconserves energy. Only the combination ofphoton frequencies that is phase matched willbe efficiently generated.

Phase matching refers to fixing the relativephase between two or more frequencies oflight as the light propagates through thecrystal. In materials, the refractive index isdependent on the frequency of light

propagating through the material.In these materials, the phaserelation between two photons ofdifferent frequencies will vary asthe photons propagate throughthe crystal, unless the crystal isphase matched for thosefrequencies. It is necessary for thephase relation between the inputand generated photons to beconstant throughout the crystalfor efficient nonlinear conversionof input photons. If this is not thecase, the generated photons willdestructively interfere with eachother, limiting the number ofgenerated photons that exit thecrystal. This is shown in the plots.Traditional phase matchingrequires that the light ispropagated through the crystal ina direction where the naturalbirefringence of the crystal has thesame refractive index as thegenerated photons. The

drawbacks to this technique include thelimited number of available materials and therange of wavelengths in those materials thatcan be phase matched.

PPLN is an engineered, quasi-phase-matchedmaterial. The term engineered refers to thefact that the orientation of the LithiumNiobate crystal is periodically inverted(poled). The inverted portions of the crystalyield generated photons that are 180° out ofphase with the generated photon that wouldhave been created at that point in the crystalif it had not been poled. By choosing thecorrect periodicity with which to flip theorientation of the crystal, the newly generatedphotons will always (at least partially)interfere constructively with previouslygenerated photons, and as a result, thenumber of generated photons will grow as thelight propagates through the PPLN, yielding ahigh conversion efficiency of input togenerated photons. The periodicity of thepoling should be such that the crystalstructure is inverted when the number ofgenerated photons at a given point in thecrystal is at a maximum as shown in the plot.

The period with which the crystal needs to beinverted (the poling period) depends on thewavelengths of the light (input and generated)and the temperature of the PPLN. Forinstance, consider a PPLN crystal that has apoling period of 6.6µm at room temperature.It will efficiently generate frequency doubledphotons from 1060nm photons when thecrystal temperature is held at 100°C. Byincreasing the temperature of the crystal to200°C PPLN will efficiently generatefrequency doubled photons from 1068.6nmphotons. Changing the temperature of thecrystal varies the phase matching conditions,which alters the periodicity of the poling inthe crystal and thereby allows some tuning ofthe generated photon frequency. Thusadjusting the temperature allows some tuningof the generated photon wavelength.

How are PPLN crystals made?

The key to producing PPLN is the process bywhich the crystal structure of LithiumNiobate is inverted (poled). Lithium Niobateis a ferroelectric crystal, which means thateach unit cell in the crystal has a small electric

dipole moment. The orientation of theelectric dipole in a unit cell is dependent onthe positions of the niobium and lithium ionsin that unit cell. The application of an intenseelectric field can invert the crystal structurewithin a unit cell and as a result flip theorientation of the electric dipole. The electricfield needed to invert the crystal is very large(~22kV/mm) and is applied for only a fewmilliseconds, after which the inverted sectionsof the crystal are permanently imprinted intothe crystal structure. To produce PPLN, aperiodic electrode structure is deposited onthe lithium niobate wafer, and a voltage isapplied to invert the crystal underneath theelectrodes. The voltage must be very carefullycontrolled so that the poled regions arecreated with the desired shape. The design ofthe electrodes is key to producing perioficity ashort PPLN crystal that can be used for anefficient SHG process, which producesphotons in the visible portion of theelectromagnetic spectrum.

Example uses of PPLN

Optical Parametric Oscillator:

One of the most common uses of PPLN is inan Optical Parametric Oscillator (OPO). Aschematic of an OPO is shown on the nextpage. The common arrangement uses a1064nm pump laser and can produce signaland idler beams at any wavelength longerthan the pump laser wavelength. The exactwavelengths are determined by two factors:

Gen

erat

ed P

hoto

ns

Distance Distance

No Phase Matching

Gen

erat

ed P

hoto

ns

Quasi-Phase Matching

Strong Beam ofGenerated Photons

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energy conservation and phase matching.Energy conservation dictates that the sum ofthe energy of a signal photon and an idlerphoton must equal the energy of a pumpphoton. Therefore an infinite number ofgenerated photon combinations are possible.However, the combination that will beefficiently produced is the one for which theperiodicity of the poling in the lithiumniobate creates a quasi-phase matchedcondition. The combination of wavelengthsthat is quasi-phase matched, and hencereferred to as the operation wavelength, isaltered by changing the PPLN temperature orby using PPLN with a different poling period.Nd:YAG pumped OPOs based on PPLN canefficiently produce tunable light atwavelengths between 1.3 and 5µm and caneven produce light at longer wavelengths butwith lower efficiency. The PPLN OPO canproduce output powers of several watts andcan be pumped with pulsed or CW pumplasers.

Second Harmonic Generation:

PPLN is one of the most efficient crystals forfrequency doubling. It has been used tofrequency double pulsed 1064nm beams withup to 80% conversion efficiency in a singlepass, thus eliminating the need for difficultlaser designs with intra-cavity doublingcrystals or matched external cavities, whichare needed with conventional doublingcrystals. The power handling is excellent forinfrared pump and output wavelengths (e.g.SHG of 1550nm → 775nm); however, whenusing PPLN to frequency double into thevisible, the power handling ability of thecrystal is more limited. It has beendemonstrated that PPLN can handle up to600mW at 532nm when frequency doubling1064nm. The exact power handling limit andconversion efficiency depend on theproperties of the laser beam used (e.g. pulselength, repetition rate, beam quality, and linewidth.)

How to use PPLN

Focusing and the Optical Arrangement:

Since PPLN is a nonlinear material, thehighest conversion efficiency from inputphotons to generated photons will occurwhen the intensity of photons in the crystal isthe greatest. This is normally accomplished bycoupling focused light into the center of thePPLN crystal through the end face of the

crystal at normal incidence. For a particularlaser beam and crystal, there is an optimumspot size to achieve optimum conversionefficiency. If the spot size is too small, theintensity at the waist is high, but the Rayleighrange is much shorter than the crystal.Therefore, the size of the beam at the inputface of the crystal is large, resulting in a loweraverage intensity over the whole crystallength, which reduces the conversionefficiency. A good rule of thumb is that for aCW laser beam with a Gaussian beam profile,the spot size should be chosen such that theRayleigh range is half the length of the crystal.The spot size can then be reduced in smallincrements until the maximum efficiency isobtained. The PPLN material has a highindex of refraction that results in a 14%Fresnel loss per uncoated surface. To increasethe transmission through the crystals, thecrystal input and output facets are AR coated,thus reducing the reflections at each surface toless than 1%.

Polarization:

The polarization of the input light must bealigned with the dipole moment of the crystalin order to utilize the nonlinear properties oflithium niobate. This is accomplished byaligning the polarization axis of the light withthe thickness of the crystal. Light polarizedorthogonal to the thickness of the crystal willbe transmitted through the crystal unaltered.

Temperature and Period:

The poling period (PP) in the crystal isdetermined by the wavelength of light beingused. The quasi-phase-matched wavelengthcan be tuned slightly by varying thetemperature of the crystal, which changes thepoling period. The Thorlabs/StratophasePPLN crystals all have multiple PP sections ineach crystal, each with a different polingperiod, which allows different wavelengths tobe used at a given crystal temperature. Theoptimum temperature can be determined byadjusting the temperature while monitoringthe output power at the generatedwavelength. PPLN is usually used attemperatures between 100°C and 200°C.

The Thorlabs/Stratophase PPLN oven is easyto incorporate into an optical setup and canstably maintain the elevated temperature ofthe crystal. Temperatures in the 100°C-200°Crange are used in order to minimize thephotorefractive effect that can damage thecrystal and causes the output beam to bedistorted. Since the photorefractive effect ismore severe in PPLN when higher energyphotons in the visible part of the spectrum arepresent in the crystal, it is especiallyimportant to use the crystal only in therecommended temperature range. Whenusing a PPLN crystal as an OPO that ispumped with and generates light in theinfrared region of the spectrum, it may bepossible to use temperatures lower than100°C if necessary without damaging thecrystal.

OPO Output MirrorHigh Transmission at Idler λ

R = 70% at Signal λ

OPO Input MirrorHigh Transmission at Pump λ

R = 100% at Signal λ

PeriodicallyPoled Regions

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Periodically Poled Lithium Niobate – Wavelength Selection Guide

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Optics

By using PPLN in combination with common lasers, it is possible to access a wide range of wavelengths in the visible and infrared. Thesewavelengths are created by either frequency doubling a laser or mixing the output of two lasers together in a PPLN crystal. Where astandard catalog PPLN crystal is appropriate, the crystal type is shown. Crystals for other applications and advice are available fromStratophase.

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Example Applications

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Frequency doubling and tripling an Erbium fiber laser

■ 83% Conversion Efficiency (1536nm → 768nm) ■ 34% Conversion Efficiency (768nm → 384nm)■ Excellent Beam Quality

Reference – “Highly efficient second-harmonic and sum-frequency generation of nanosecond pulses in a cascaded erbium-doped fiber: periodically poled lithium niobate source”, D.Taverner, et al.Optics Letters Vol. 23, (3), 162-164, (1998).

Fiber laser, 1536nm10ns pulses

PPLNCrystal

PPLNCrystal

1536nm 384nm768nm

Residual1536nm

High Power 473nm generation

■ 450mW Average Power at 473nm■ 40% Conversion Efficiency■ No Observed Photorefractive Effect■ Excellent Beam Quality

Reference – “Generation of high-power blue light in periodically poled LiNbO3”, G.W.Ross et al. Optics Letters 23, (3) 171-173 (1998).

946nm diode pumpedNd:YAG, 300ns pulses

PPLNCrystal

946nm 473nm

Generation of picosecond, tunable, infrared light

■ 4ps Pulses Tunable IR Light, 1.3 – 5.3µm■ Signal Power: >400mW■ Slope Efficiency, Signal and Idler up to 160%■ Threshold: 7.5mW

Mode locked 1047nmNd:YLF, 4ps pulses

PPLNCrystal

1047nm 1.3 – 5.3µmSignal & Idler

Reference – “Efficient, low-threshold synchronously-pumped parametric oscillation in periodically-poled lithium niobate over the 1.3µm to 5.3µm range" L.Lefort et al. Optics Communications152, (1-3), 55-58 (1998).

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PPLN For Difference Frequency GenerationDifference Frequency Generation (DFG) allows tunable pump sources to be used toproduce widely tunable outputs that are ideally suited to a variety of spectroscopicapplications. The accessible range of frequencies that can be generated using ourDFG PPLN crystals is further increased due to the inclusion of 13 optical paths,each with a different poling periodicity. Each optical path can be temperature tunedto provide a range of generated photon frequencies.

■ Components are Available From Stock■ High Efficiency Wavelength Conversion■ Mounted Crystals for Alignment-Free Insertion Into Temperature Controlled

PPLN Oven■ Other Crystals Available (e.g. Alternative Lengths, Periods, and

AR Coating Specifications)

Specifications■ Input Face AR Coated, R<1%

at Specified Pump Wavelengths■ Output Face AR Coated, R<1%

at Specified DFG Wavelengths■ Polished to 20-10 Scratch-Dig■ Flatness: <λ/4 @633nm■ Fewer Than 2 Edge Chips Larger Than

100µm Per Face■ Faces Parallel to Within ±5 Minutes

PPLN Crystal used to Generate Tunable Light from 2.44-3.17µm

0.5mm thick PPLN crystal designed for DFG of wavelengths in the range of 2.4 – 3.2µm, using simultaneous pump wavelengths from both tunableTi:Sapphire and Nd:YAG lasers.

■ Suitable for Producing Wavelengths From 2.4 – 3.2µm

ITEM APPLICATION PERIODS CRYSTAL OPERATINGLENGTH (mm) TEMPERATURE $ £ € RMB

DFG1-20 DFG of 2.44 – 3.17µm Using Simultaneous 18.00, 18.25, 18.50, 18.75,20 160 - 200°C $2,350.00 £1,480.50 €2.185,50 ¥22,442.50

Pumps at 742 – 796nm and 1064nm. 19.00, 19.25, 19.50, 19.75,DFG1-40 DFG of 2.44 – 3.17µm Using Simultaneous 20.00, 20.25, 20.50,

40 160 - 200°C $2,950.00 £1,858.50 €2.743,50 ¥28,172.50Pumps at 742 – 796nm and 1064nm. 20.75, and 21.00µm

PPLN Crystal used to Generate Tunable Light from 2.85–4.74µm

0.5mm thick PPLN crystal designed for DFG of wavelengths in the range of 2.84 – 4.74µm, using simultaneous pump wavelengths from both tunableTi:Sapphire and Nd:YAG lasers.

■ Suitable for Producing Wavelengths From 2.85 – 4.74µm

ITEM APPLICATION PERIODS CRYSTAL OPERATING LENGTH (mm) TEMPERATURE $ £ € RMB

DFG2-20 DFG of 2.85 – 4.74µm Using Simultaneous 20.00, 20.25, 20.50, 20.75,20 160 - 200°C $2,350.00 £1,480.50 €2.185,50 ¥22,442.50

Pumps at 775 - 869nm and 1064nm. 21.00, 21.25, 21.50, 21.75,DFG2-40 DFG of 2.85 – 4.74µm Using Simultaneous 22.00, 22.25, 22.50,

40 160 - 200°C $2,950.00 £1,858.50 €2.743,50 ¥28,172.50Pumps at 775 - 869nm and 1064nm. 22.75, and 23.00µm

PeriodicallyPoled Region

13 PeriodicallyPoled Regions

Length

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