Continuous-wave, singly-resonant parametric ...

Continuous-wave, singly-resonant parametric oscillator based mid-infrared optical vortex source A. AADHI,* VARUN SHARMA, R. P. SINGH, AND G. K. SAMANTA 1Photonic

Sciences Lab, Physical Research Laboratory, Navarangpura, Ahmedabad 380009, Gujarat, India

*Corresponding author: [email protected] Received XX Month XXXX; revised XX Month, XXXX; accepted XX MonthXXXX; posted XX Month XXXX (Doc. ID XXXXX); published XX Month XXXX

We report on a high power, continuous-wave source of optical vortices tunable in the mid-infrared wavelength range. Using the orbital angular momentum (OAM) conservation of the parametric processes and the threshold conditions of the cavity modes of the singly resonant optical parametric oscillator (SRO), we have transferred the OAM of the pump beam at near-infrared wavelength to the idler beam tunable in the mid-infrared. Pumped with vortex beam of order, lp=1 at 1064 nm the SRO, configured in a four curved mirrors based ring cavity with a 50 mm long MgO-doped periodically poled LiNbO3 crystal, produces idler beam with output power in excess of 2 W in vortex spatial profile with order, li=1, tunable across 2217-3574 nm and corresponding signal beam in Gaussian intensity distribution across 15152046 nm. For pump vortices of order, lp=1 and 2, and power of 22 W, the SRO produces idler vortices of same order as that of the pump beam with a maximum power of 5.23 W and 2.3 W corresponding to near-IR to mid-IR vortex conversion efficiency of 23.8% and 10.4% respectively. The idler vortex beam has a spectral width and the passive rms power stability of 101 MHz and 4.9% over 2 hours, respectively. ©2017 Optical Society of America OCIS codes: (190.4970) Parametric oscillators and amplifiers; (190.4410) Nonlinear optics, parametric processes; (260.6042) Singular optics; (190.4223) Nonlinear wave mixing; (140.3510) Lasers, fiber; (190.7110) Ultrafast nonlinear optics. http://dx.doi.org/10.1364/OL.99.099999

The phase singularity of optical beams is defined by a point on the wavefront where the phase is undefined [1]. Such undefined phase results in vanishing field amplitude at the singular point and make the optical beams carry a doughnut shaped spatial intensity profile. Such phase singularities are described by the azimuthal phase variation of the electric field exp(±il𝜃), where, the real integer, l, and 𝜃 are the topological charge or the order of the vortices and the azimuthal angle respectively. The sign, (±), indicates the direction of azimuthal phase variation of vortex beams. The optical vortices of order, ±l, carry the orbital angular momentum of ±lћ per photon [2]. Over decades, optical vortices have acquired considerable interest due to their wide variety of applications in science and technology including quantum information [3], optical communication [4], particle micromanipulation and metrology [5], material processing [6] and

nonlinear optics [7,8]. Typically, optical vortex beams in the visible and near-infrared wavelengths have been used extensively in most of the applications. However, the optical vortices in the mid-infrared wavelength region, the fingerprint absorption bands of numerous molecules, have not been explored so far. This is mainly due to the difficulties in generating optical vortices in these wavelength ranges using conventional mode converters [9-13] including spiral phase plates spatial light modulators and S-waveplates. However, the future advancement in super-resolution molecular spectroscopy, light induced chiral structure formation in organic molecules, and understanding chiral metamaterials, demand generation of optical vortex beams in the mid-infrared wavelength range. Due to the unavailability of suitable mode converters, nonlinear optics has emerged as an alternative tool to generate optical beams of various spatial structures [14-17] over wide wavelengths. In recent times, efforts have been made to generate optical vortices in mid-infrared region through the frequency conversion of high energy nanosecond [18,19] and femtosecond [8] near-infrared lasers of vortex spatial profile. While the generation of nanosecond and femtosecond vortices at mid-IR wavelengths are easily realized in optical parametric oscillators (OPOs) and optical parametric generators (OPGs), the low parametric gain under continuous-wave (cw) pumping have so far been the most challenging factor in generating tunable cw vortex beams. To overcome such problem, we have recently used doubly resonant OPO (DRO) and demonstrated tunable vortices with controlled OAM transfer among the interacting beams [20]. However, the DRO based vortex sources suffer from the common drawback of limited wavelength tunability, phase instability and power fluctuations. On the other hand, singly resonant OPO (SRO) offers the most viable solution for high-power optical radiation in good spatial and spectral quality over extended spectral regions. Therefore, the SROs can be the most appropriate choice to generate optical vortices in a simple and compact system architecture. However, the SRO has highest operation threshold as compared to other OPO configurations. Additionally, the threshold of SRO increases for pump beam other than Gaussian beam. In recent times, the use of improved periodically poled crystals pumped with high power fiber lasers have made cw SRO operation even in the presence of substantial cavity losses [21]. Therefore, direct generation of vortex beam from the SROs can be a major step for the realization of a compact high-power source of structured spatial beams with wide wavelength tunability. Here, we demonstrate a cw SRO producing high power optical vortex tunable in the mid-IR wavelength range. Transferring the pump vortex at 1064 nm to the idler beam in a SRO we have generated optical vortex beam continuously tunable across 2217–3574 nm (1357 nm) with maximum vortex power of 5.23 W and 2.3 W at vortex orders, li=1, and li=2 respectively. To the best of our knowledge, this is the first report on cw SRO producing high power optical vortex in the mid-IR wavelength range.

Fig. 1. Schematic experimental setup for the vortex pumped SRO. λ/2, half-wave plate; SPP, spiral phase plate; L, lens; C, MgO:PPLN nonlinear crystal; M1-4, mirrors; S, wavelength separator. The schematic of the experimental setup is shown in Fig. 1. The pump source is a cw Yb-fiber laser (IPG Photonics, YLR-50-1064-LP-SF), delivering single-frequency (linewidth of 99.5%) for the signal radiation across 1400–2000 nm, and high transmission (T>90%) at pump and idler radiation across 2100–4000 nm to ensure SRO operation. A 50-mm long, 8-mm-wide, 1-mm-thick, multi-grating MgO-doped periodically poled LiNbO3 (MgO:PPLN) crystal is used for OPO operation. The crystal contains seven gratings of periods, Λ=28.5–31.5 μm, with an increment of 0.5 μm steps. The end faces of the crystal are antireflection coated for the resonating signal (R2.

Fig. 2. (a-b) Far field intensity distribution, and (c-d), corresponding lobe structure of the pump vortices of orders, lp=1 and lp=2. (e-f) Farfield intensity distribution, (g-h) lobe structure of the idler beam, and the (i-j) intensity distribution of the signal beam generated by the pump vortex of orders, lp=1 and lp=2. After confirming the transfer of pump vortex to the idler beam in the SRO, we have studied the generation of vortex beam tunable across the mid-IR spectrum. Pumping the SRO with the vortex beam at constant power of 20 W we have measured the intensity profile of the idler beam while varying the crystal temperature and grating period across 30-200oC and 29.5-31.5 µm, respectively. The results are shown in Fig. 3. As evident from the first column, (a)-(b), of Fig. 3, the intensity profile and the corresponding lobe structure shows the pump beam has

higher beam divergence at longer wavelengths. It is also interesting to note that more than 95% (1290 nm) of the tuning range have output power in excess of 2 W in a stable vortex spatial profile. While the overall wavelength tunability of the vortex beam can be increased by tuning the SRO close to degeneracy with the use of proper coating of the cavity mirrors, the tuning of idler vortex beyond 3574 nm requires adjustment in the beam divergence through proper selection of pump beam focusing and cavity configuration. The pump depletion of the vortex SRO across the tuning range as shown in Fig. 4(b), varies from 55% at 2217 nm to 37% at 3574 nm with a maximum pump depletion as high as ~73.5 % at an idler wavelength of 2695 nm. Almost entire tuning range has the pump depletion of >50%.

Depletion (%) Idler Power (W)

vortex order of lp=1. For such pump beam, the second and third column, (c)-(f), and (g)-(j), of Fig. 3 show the intensity profile of the signal and idler beam respectively. As evident from the second column, (c)-(f) of Fig. 3, the signal beams across the wavelength range of 15152012 nm have Gaussian intensity distribution (vortex order of ls=0). However, the corresponding idler beams across 3574-2258 nm, as shown by the third column (g)-(j) of Fig. 3, have doughnut shaped intensity profiles. The number of lobes of the idler beam recorded at the focal point of the tilted lens as shown by fourth column, (k)-(n) of Fig. 3, confirm the order of the idler vortices to be li=1 and satisfy the OAM conservation, lp=ls+li, throughout the tuning range of the OPO. In contrary to the asymmetric and unstable intensity profile of the mid-IR vortices generated through the doubly resonant OPO [19], here the use of SRO results in highly stable symmetric optical vortices (see third column of Fig. 3) continuously tunable over 1316 nm in the mid-IR wavelength region across 2258-3574 nm. Such high stability in the spatial profile of the idler vortex can be attributed to the intrinsic stability of the cavity modes, and the transfer of pump mode to the nonresonant beam (here idler) of the SRO. This is so far the first report on generation of high-quality cw, optical vortices tunable over such a broad wavelength range.

6

2 0 80

Pump = 20 W = 29.5 - 31.5 m

li = 1

4 >2W (a)

> 50 % 40 (b) 2400 2800 3200 Idler Wavelength (nm)

3600

Fig. 4. Variation of (a) idler vortex power, and (b) pump depletion across the tuning range of the SRO for the pump vortex of order, lp=1. The lines are guide to eyes.

Fig. 3. (a) Far-field intensity distribution and (b) lobe structure of the pump beam of vortex order, lp=1. Far-field intensity distribution of the Gaussian signal beam (c-f) and corresponding vortex idler beam (g-j) generated for the pump vortex of order, lp=1. (k-n) Lobe structure of the idler beam measured using the tilted lens technique. With the successful generation of optical vortices tunable across mid-IR wavelength range, we have measured the idler power and pump depletion across the tuning range. The results are shown in Fig. 4. As evident from Fig. 4(a), for the pump vortex of order, lp=1, at constant power of 20 W, the idler vortex power varies from 4.32 W at 2217 nm to 1.62W at 3574 nm, with a maximum power of 4.7W at 2310 nm with vortex-vortex conversion efficiency as high as 23.5%. The steady fall of idler power at longer wavelengths can be attributed to the decrease of parametric gain away from degeneracy and lower mode overlapping of the idler vortex beam with the interacting beams due to

We have also measured the power scaling characteristics of the SRO for pump vortices of orders, lp=1 and lp=2. Operating the SRO at an idler wavelength of 2310 nm by using the grating period and crystal temperature 31.5 µm and 190°C respectively, we have recorded the idler power with the vortex pump power. The results are shown in Fig. 5. As evident from Fig. 5(a), the idler power for the pump vortex of order, lp=1, increases linearly to the pump from a threshold power of ~8.5 W with a slope efficiency of 33.9%. We have recorded a maximum vortex idler power of 5.23 W for the pump power of 22 W at a vortexvortex conversion efficiency as high as 23.8%. On the other hand, under the same experimental condition, the pump vortex of order, lp=2, as evident from Fig. 5(b), produces a maximum idler power of 2.3 W for the pump power of 22 W at relatively low slope efficiency (~28%) and vortex-vortex conversion efficiency (~10.4%). For the pump vortex of order, lp=2, the SRO has an operational threshold of ~15.2 W, almost 1.8 times higher than that of the pump vortex of order, lp=1. Such higher operational threshold and the lower conversion efficiency can be attributed to the reduction in parametric gain due to the lower overlap integral of the interacting beams along the nonlinear crystal with vortex order. In addition to the high power vortex idler beam, the finite coupling of the cavity mirrors at the signal wavelength 1972 nm, results in a usable out-coupled leakage signal beam of power of 245 mW and 87 mW in Gaussian spatial profile for the pump vortex of order, lp=1, and lp=2, respectively. Using the transmission (~0.2%) of the cavity mirrors we have estimated the intra-cavity signal powers to be ~122W and ~43W for lp=1, and lp=2, respectively. As such, one can extract signal beam of considerable output power in the Gaussian profile by using proper output coupler to the SRO, however, the increase in the operation threshold can be the major concern especially for higher order pump vortices. Since there is no sign of saturation

effect in the idler power, one can in principle, increase idler vortex power with higher power pump. However, in the present experiment, we have restricted to operate below 25 W of pump power to avoid possible crystal damage.

6

3

i = 2310 nm

i = 2310 nm

Idler Power (W)

lp = 1

lp = 2

4

2

(a)

(b)  = 28 %

2

1

References

 = 33.9 %

0 0 10 20 30 Pump Power (W)

0 0

10 20 30 Pump Power (W)

Fig. 5. Variation of idler power with pump power for the vortex order, (a)lp=1, and (b)lp=2. Lines are linear fit to the experimental results. Further, we have measured the power fluctuation of the idler beam of vortex order, li =1, with the results shown in Fig. 6. As evident from Fig. 6, the idler beam at 3548 nm has a passive rms power fluctuation of 4.9% over 2 hours. Using suitable isolation of the SRO from the temperature fluctuation and air turbulence of the laboratory environment, one can in principle improve the power stability of the vortex idler beam. Additionally, we have measured the linewidth of the idler radiation at 2310 nm for li = 1 using a scanning confocal Fabry– Perot interferometer (FPI, free spectral range, FSR~ 10 GHz, Finesse 150). The results are shown in the inset of Fig. 6. The equidistant interference fringes of the FPI as shown in the inset of Fig. 6 confirm the idler radiation to have single longitudinal mode with an instantaneous linewidth of ~101 MHz. We have observed similarly single-frequency performance of the idler radiation across the entire tuning range.

2.0

i= 3548 nm

RMS Stability =4.9

1.5

0.5 0.0

-0.5 0.0

2  = 2310 nm i 10 GHZ 1

45

l=1

Ramp Voltage (V)

1.0

Idler Power (a.u.)

Idler Power (W)

with vortex beam, we observe that in cw SRO, the spatial profile of the pump beam is transferred to the non-resonant beam whereas the resonant beam retains a Gaussian profile due to the threshold condition of the cavity modes. For pump vortex orders, lp=1 and lp=2, the SRO generates idler beam with same vortex order as that of the pump with a maximum output power (vortex-vortex conversion efficiency) of 5.23 W (23.8 %) and 2.3 W (10.4 %) respectively for 22 W of pump power. Using the pump vortex of order, lp=1, we have generated idler of vortex order, li=1 with output power >2 W over the tunable wavelength range across 2217-3574 nm with maximum idler power of 4.7 W for 20 W of pump power at 2310 nm. The single frequency idler vortex radiation has an instantaneous linewidth of 101 MHz and passive rms power stability better than 4.9 % over 2 hours.

30

101MHZ 15 0 0

0.5

5 10 15 Time (ms)

1.0 Time (Hrs)

0 20

1.5

2.0

Fig. 6. Stability of the idler vortex power. (Inset) Transmitted interference fringe of the Fabry–Perot interferometer at idler wavelength of 2310 nm. In conclusion, we have demonstrated a high power cw OPO providing optical vortex beam tunable in the mid-IR wavelength range. Pumping

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