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BBO Crystals and Its Applications

Introduction

Beta-Barium Borate, β-BaB2O4 or BBO Crystal, is an efficient NLO crystal material first invented by FIRSM, CAS (Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences) i n 1979. Among the family of nonlinear optical crystals, BBO crystal is a member with obvious comprehensive advantages and superior benefits, featuring a wide light transmission range,  weak piezoelectric ringing effect, and a large nonlinear optical coefficient.  Due to the above characteristics, BBO crystals are a capable candidate for SHG, THG, 4HG, and 5HG, of Nd: YAG crystals, SHG, THG, 4HG of Ti: Sapphire and Alexandrite lasers, optical parametric amplifiers (OPA) and optical parametric oscillators (OPO), second harmonic generation of argon ion, ruby, and Cu vapor lasers, and the cutting-edge research and development fields of laser technologies such as all-solid-state tunable lasers, ultrafast pulse lasers, and deep ultraviolet lasers. It is recognized as one of the excellent second-order nonlinear optical crystals in the world with unrivaled performance.  In addition, when compared with other crystals intended for electro-optic modulation, it excels in a higher extinction ratio, larger phase matching angle, higher damage threshold, and excellent optical homogeneity, which is conducive to stable laser output power.


The merits of BBO Crystals include:

  • Wide phase-matching range: 409.6~3500 nm;
  • Extensive light transmission range: 190~3500 nm;
  • Large second harmonic generation coefficient: about 6 times that of KDP crystal;
  • High laser damage threshold: about 10 GW/cm2 (100 ps width, 1064 nm)
  • High optical homogeneity: δn=10-6
  • Wide temperature bandwidth: about 55 ℃.



BBO Crystals from Shalom EO


BBO and Nonlinear Frequency Conversion

Nonlinear Frequency Conversion is a measure to leverage optical nonlinearity for converting part of the optical power of some input light to output light in a different wavelength region. The input light source is often quasi-monochromatic light with a fixed wavelength or sometimes a tunable wavelength; during some occasions, broadband light is produced. Nonlinear Frequency Conversion only occurs when laser beams of a substantial optical intensity are incident on the media. The output is then also often created in the form of a laser-like beam, i.e., with high spatial coherence.

Frequency conversion could be categorized into second harmonic generation (Click here to learn more about What is Second Harmonic Generation) and a three-wave nonlinear process where two pump beams generate another beam with the sum or difference of the optical frequencies of the pump beams. Second-harmonic generation is a special case in which two photons of the same frequency are combined into a single photon of twice the frequency. Three-wave nonlinear optical processes include difference-frequency (DFG) generation, sum-frequency generation(SFG), optical parametric oscillation, and optical parametric amplification.

The purpose of nonlinear frequency conversion is to obtain wavelength regions of interest that are not naturally accessible in the emission spectrum of lasers. Therefore, it is common e.g. to generate visible or ultraviolet light by nonlinear conversion of near-infrared light from one or several lasers. Also, mid-infrared laser sources are often realized based on a near-infrared laser combined with some nonlinear frequency conversion apparatus. Nonlinear frequency conversion can only take place when phase matching condition is met.


BBO and Harmonic Generations

Harmonic generation is a non-linear optical process in which photons of intense incoming laser radiation interact with a non-linear material and radiation with corresponding harmonics frequencies is generated. This often occurs at optical intensities of the order of 1014 W/cm2 or higher. When n photons of identical frequencies interact with a nonlinear material, the photons are "joined", and generate a new photon which contains n times the energy of the initial photons.

BBO for Second, Third, Fourth, and Fifth Harmonic Generation of Nd: YAG Lasers:

Exceptional conversion efficiencies could be achieved using BBO crystals, which are greater than 70% (SHG), 60% for THG, 50% for 4thHG. BBO crystal is the best option for 213nm 5HG.

BBO crystals excel in the ability to handle intracavity SHG of Nd:YAG lasers with high power densities to generate 532nm output. And 266nm output with a decent power level could be attained using a Brewster-angle-cut BBO crystal when pumping the Nd:YAG Laser with an SHG output of a mode-locked Nd:YLF laser.

Because of a small acceptance angle and large angular walk-off, good laser beam quality (small divergence, good mode condition, etc.) is the crucial factor for BBO to function better, tight focusing of laser beams is not recommended.

BBO for Harmonic Generations of Other Tunable Lasers:

  • Ultra-fast Lasers:

For SHG and THG of ultra-fast Lasers, BBO also exhibits a great advantage over KDP and ADP crystals. 

  • Dye Lasers

BBO crystal is an effective media for SHG of XeCl-laser pumped Dye lasers for giving output in the ultra-violet spectrum (205-310nm).

  • SHG/THG of Alexandrite Lasers and SHG/THG/FHG of Ti:Sapphire Lasers

SHG/THG of Alexandrite Lasers UV output in the region 360nm-390 nm and output in the region 244nm - 259 nm. SHG/THG/FHG of Ti:Sapphire Lasers could also be realized efficiently.

  • Intracavity SHG of Argon Doped Lasers and SHG of Copper-Vapor Lasers

Intracavity SHG of Argon Doped Lasers for UV and Deep-UV output, and SHG of Copper-Vapor Lasers for UV output.


BBO for OPO and OPA

Optical Parametric Amplification (OPA) is a process in which a high-power laser beam of circular frequency ωp (referred to as pump light) enters the nonlinear crystal, resulting in gains at two lower frequencies (ωs and ωi, and ωs+ωi=ωp) and amplification of the beams of corresponding frequencies. The gain is maximized when the phase-matching condition is met. During the course, a beam of frequency ωs (signal light) is incident on the nonlinear crystal, its light intensity will be amplified, and coherent light (idle light) will be generated at the frequency ωi when no beam is incident. From the perspective of photonics, this is a process in which the photons of the pump light are annihilated to generate two low-frequency photons (corresponding to signal light and idle light, respectively).

Optical Parametric Oscillation (OPO) is a process in which an input laser wave (called "pump") with frequency ωp is converted into two output waves of lower frequency (ωs, ωi) by means of second-order nonlinear optical interaction.The sum of the output waves' frequencies is equal to the input wave frequency ωs+ωi=ωp. The two output waves are addressed as "signal" and "idler", where the output wave with higher frequency is the "signal". If a nonlinear medium is placed in an optical cavity, with the pump light wave, signal light wave, and idle light wave traveling through the medium in repeat, when the gain of the signal light wave and idle light wave due to parametric amplification is greater than their perception in the cavity, laser oscillation will be formed in the cavity, that’s the working principle of Optical Parametric Oscillation.

BBO for Optical Parametric Oscillation (OPO)/Optical Parametric Amplification (OPA):

Utilizing BBO crystals for optical parametric oscillation(OPO) and parametric optical amplification (OPA), one could easily obtain tunable output in a broad wavelength range (from the UV to IR). Common applications include: 

  • OPO of 532nm pumped lasers for output ranging from 680 nm to 2400 nm

our BBO Crystals could support OPO output of 532nm pumped laser with decent conversion efficiencies. 

  • OPO and OPA of 355nm Nd:YAG pumped lasers

Using Shalom EO’s BBO, OPO of 355nm Nd:YAG pumped lasers could produce a wide tunable approximate wavelength range from UV to the MWIR spectrum, with promising conversion efficiencies.

OPA of BBO crystals generates narrow-band, high power, and tunable (400 nm to 2000 nm) pulses for a pico-second Nd-YAG pumped laser. The approach is way better than using Dye Lasers in terms of efficiencies, tunable range, and maintenance.


BBO and Electro-optical Modulations (EOM)

Electro-optical Modulation (EOM) is a process in which control of power, phase pr polarization of light is realized by adjusting an electrical signal. The modulation is based on the electro-optical effect which could be defined as the change of absorption and the change in the refractive index of a material, resulting from the application of a DC or an electric field with much lower frequencies than the optical carrier. A nonlinear crystal could be viewed as the simplest form of an electro-optical modulator, whose refractive index is a function of the strength of the local electric field.

Beta BBO crystals or beta barium borate crystals exhibit significant advantages over other materials for EOM application in terms of laser power handling abilities, high damage threshold temperature stability, and substantial freedom from piezoelectric ringing. It is an excellent electro-optic crystal for high-power applications at the wavelength range from 200nm to 2,500nm. Beta BBO crystals are the most attractive candidates for high repetition rate Q-switching, pulse picking at up to 3 MHz, laser cavity dumping, regenerative amplifier control, and beam chopper. 



BBO Crystals for EO Applications from Shalom  EO.


BBO and Spontaneous Parametric Down Conversion (SPDC)

In quantum optics, Spontaneous Parametric Down Conversion (SPDC) is a critical technology that can be utilized to prepare individual photons or photon pairs that are quantum-entangled with each other. 

Back in 1970, David Burnham and Donald Weinberg first gave a detailed scientific description of spontaneous parametric down-conversion.

The basic concept is using a nonlinear crystal to split a photon into a photon pair, where the original photon is called a "pump photon", and the two photons in the photon pair are called "signal photon" and "idle photon". According to the law of conservation of energy and the law of conservation of momentum, the total energy and total momentum of the photon pair are equal to the energy and momentum of the pump photon. From the law of conservation of energy, we can get ωp=ωs+ωi, Where ωp, ωs, ωi are the angular frequencies of pump photons, signal photons, and idler photons.

From the law of conservation of momentum, we can get kp=ks+ki, Where kp, ks, ki are the wave number vectors of pump photons, signal photons, and idle photons.

The two equations stated above are called phase-matching conditions. Only certain types of nonlinear crystals can achieve this condition, for example, barium metaborate crystals or potassium dihydrogen phosphate crystals.

If the signal photon and the idler photon share the same polarization and are perpendicular to the pump photon, this is called type 1 phase matching; if the polarization of the signal photon and idler photon are perpendicular to each other, it is called type 2 phase matching. Pairs of photons emitted successively do not have any polarization correlation with each other.

At present, the method of using nonlinear optical crystals, in particular BBO crystals, to prepare high-brightness entangled photon pairs has become mainstream to prepare quantum entanglement and studies about Bell Inequalities. This photon source has significant advantages. First, it is simple in structure, convenient to implement and detect; second,, it can generate two very small correlated photon beams, which can be input into a large length of optical fiber or transmitted in free space. Therefore, the light source and the measurement device are allowed to be separated by a long distance, which has exceeded thousands of kilometers, making the verification experiment more direct and objective.

In 2022, Chinese scientists realized the high-dimensional Bell inequality test without detection loopholes for the first time. Chuanfeng Lee and Biheng Liu’s research group, led by Academician  Guangcan Guo of the University of Science and Technology of China, used BBO crystals to generate entangled photons, which increased the overall detection efficiency of high-dimensional entangled photons to 71.7% , thus accomplishing the high-dimensional Bell inequality test without detection loopholes.


Conclusion

With versatile functions and distinguished capabilities in multiple applications, BBO Crystals have proved its high commercial value and promising prospect in the global market and the field of advanced scientific research, where Shalom EO is always willing to provide the BBO crystals of top-class qualities and step forward into the zone of future technologies with you.

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