Sami T. Hendow

Structuring materials with nanosecond laser pulses

Ablation of silicon and metals is investigated using a 1064 nm pulsed fiber laser, with pulse energy up to 0.5 mJ, peak powers up to 10 kW, and pulse widths from 10 to 250 ns. A simple thermal model is employed to explain the dependence of scribe depth and shape on pulse energy or peak power. We demonstrate that pulses of high peak powers have shallow penetration depths, while longer pulses with lower peak powers have a higher material removal rate with deeper scribes. The key parameter that enables such variation of performance with changes in peak pulse power or peak irradiance on the material surface is the nonlinear increase of the absorption coefficient of silicon or metals as its temperature increases.

The role of population pulsations in single-mode laser instabilities☆

Abstract This paper gives three principal results: One, population pulsations are responsible for both the multiwavelength and the chaotic single-wavelength instabilities of single-mode operation in unidirectional ring lasers containing homogeneously-broadened media. Two, in appropriate limits the Fourier expansion method for treating three-frequency operation in saturation spectroscopy is equivalent to the linear stability analyses in laser theory and optical bistability. Three, inclusion of population pulsations significantly increases the instability range over that predicted by Casperson for single-mode bad-cavity lasers containing inhomogeneously-broadened media.

Recursive numerical solution for nonlinear wave propagation in fibers and cylindrically symmetric systems

An efficient and compact recursive numerical solution of the wave equation is developed and applied to cylindrically symmetric optical systems. Numerical results are given for wave propagation through an aperture and in linear and nonlinear optical fibers. This code is most useful for multiwave mixing and wave propagation in nonlinear media.

Theory of single-mode laser instabilities

We use the semiclassical strong-signal theory of the laser to predict and explain the onset of side-mode buildup in lasers with one oscillating mode. Two general categories are considered: one for which the side modes and the oscillating mode all have the same wavelength and the other for which they have different wavelengths. The treatments include an arbitrary amount of inhomogeneous broadening. Our approach unifies the treatments of the side-mode instabilities presented earlier and extends them to handle standing waves in addition to the previously treated running waves. We write the field and the population matrix elements as Fourier series in the adjacent-mode beat frequency. This approach has been used extensively in both multimode laser theory and saturation spectroscopy. This technique coincides with linear stability analyses used by others, provided that our beat frequency includes a contribution that is proportional to the complex side-mode gain. We give a solution that allows for detuned operation along with its simpler, centrally tuned special case. The connection with saturation spectroscopy clearly reveals that the side-mode instabilities require side-mode gain. For the single-wavelength case, nonlinear anomalous dispersion is also required. The side-mode gain and dispersion result from both inhomogeneous broadening and population pulsations. The lowest instability thresholds occur when both of these mechanisms play a role. The approach can also be used to treat instabilities in optical bistability by substituting the appropriate equation of state for the strong-mode intensity and by changing the sign of the absorption coefficient. In homogeneously broadened, standing-wave lasers, we show that multiwavelength instabilities depend strongly on the position of the medium in the cavity. We illustrate the theory by giving numerical results for the output pulsation frequency and for the instability threshold by using parameters that are appropriate for the He–Xe laser. These results correlate well with experimental observations.

Effects of detuning on single-mode laser instabilities

Abstract Linear stability analysis is used to show the effects of detuning on single-mode instabilities in unidirectional ring lasers. We find two regions of single-wavelength instability in homogeneously-broadened lasers. The first is at line center, for which population pulsations are solely responsible, and the second is off line center where the unsaturated medium provides the required gain and anomalous dispersion. Four cases are considered numerically: homogeneous and inhomogeneous broadening each with a single wavelength and with multiwavelengths.

Four-Channel, High Power, Passively Phase Locked Fiber Array

We demonstrate passive phasing in a four channel high power passively phase-locked Yb fiber laser array. We achieved an output power of 710W with high fringe visibility from an array of LMA Yb fiber lasers.

Percussion drilling of metals using bursts of nanosecond pulses

The effect of ns bursting on percussion drilling of metal is investigated experimentally and analytically, and compared with the efficiency and quality of drilling using single ns pulses. Key advantages are demonstrated, correlating well with the results from a thermal theoretical model. The 1064 nm bursts contain up to 14 pulses of various pulse widths and spacing, and at frequencies of tens of MHz within the burst. The individual pulses have pulse widths of 10 to 200 ns, and up to 12 kW peak power. Burst repetition frequency is single shot to 500 kHz.

Observation of bistable behavior in the polarization of a laser

The output of a single-mode He-Ne laser with internal mirrors is usually linearly polarized in one of two orthogonal directions. This polarization state may be selected by a tunable injected signal or by feedback from a tunable secondary optical resonator. The laser will remain in the selected polarization even after the injected signal or the feedback from the secondary resonator is removed. This effect has potential applications as a fast switch or as an optical memory. An explanation based on intracavity birefringence is given for this bistable behavior in the polarization.

Single-mode operation of Doppler-broadened lasers by injection locking

A multimode He-Ne laser can be forced to oscillate in a single mode by injecting a narrow-band signal into the resonator. We show experimentally that the minimum injected intensity required for single-mode operation occurs at an injection frequency detuned from line center. This is in agreement with the results from a semiclassical theory of an inhomogeneously broadened injection-locked laser.

MOPA PULSED FIBER LASER WITH CONTROLLED PEAK POWER AND PULSE ENERGY FOR MICROMACHINING OF HARD MATERIALS

A high brightness 1064nm MOPA pulsed fiber laser with controlled pulse width, peak power, and pulse energy is used to investigate the effects of changing the peak power and pulse width on the ablation process. Pulses of 10ns to 250ns are applied, with peak powers up to 10kW while maintaining average power. This corresponds to over 3 GW/cm2 fluence levels. The materials used in these experiments are mono and polycrystalline silicon, and aluminum coated silicon. Experimental results indicate that the material removal rates of a short pulse with high peak power is small, and that the material ablation benefits from a leading peak pulse shape at longer pulses of 100 to 150ns. These longer pulses accelerate the material removal by first heating and subsequently melting the substrate material. Material removal rate of about 13µm per scribe, single-pass, is obtained on mono silicon for 10W incident beam, 1m/s scan speed, 100 kHz rep rate and 20 µm spot size with 50% spot overlap. Percussion drilled holes are also demonstrated for 200 µm thick polycrystalline silicon wafer. Through holes are drilled in 1msec using pulses of >50ns. Entry and exit diameters are typically 30 and 20µm, respectively.A high brightness 1064nm MOPA pulsed fiber laser with controlled pulse width, peak power, and pulse energy is used to investigate the effects of changing the peak power and pulse width on the ablation process. Pulses of 10ns to 250ns are applied, with peak powers up to 10kW while maintaining average power. This corresponds to over 3 GW/cm2 fluence levels. The materials used in these experiments are mono and polycrystalline silicon, and aluminum coated silicon. Experimental results indicate that the material removal rates of a short pulse with high peak power is small, and that the material ablation benefits from a leading peak pulse shape at longer pulses of 100 to 150ns. These longer pulses accelerate the material removal by first heating and subsequently melting the substrate material. Material removal rate of about 13µm per scribe, single-pass, is obtained on mono silicon for 10W incident beam, 1m/s scan speed, 100 kHz rep rate and 20 µm spot size with 50% spot overlap. Percussion drilled holes are als...