HIGH POWER CONTINUOUS-WAVE 1064 NM DPSS LASER FOR MACHINING SEMICONDUCTOR AND METAL MATERIALS

A diode-pumped solid-state (DPSSL) laser system with 808 nm laser as pump source has been developed successfully. We used the optically anisotropic crystal Nd:YVO4 as the active medium. The threshold pump power and slope efficiency were measured and discussed. With lowly doped crystal Nd:YVO4 0.27% and concave-plane cavity, the laser showed good performance in the pumping range up to 11 W. Using the 1064 nm beam, micromachining were successfully conducted upon some normal materials such as plastic, wood; some semiconductors such as silicon and metals such as aluminum, copper, steel.


INTRODUCTION
In the late 1980s, laser diode at 808 nm with reasonable price made its first debut on the market and many scientists turned their attention to it in searching for an alternative pump source for Nd:YAG (yttrium aluminum garnet doped with neodymium) and other Ndhosted laser.Previously, the main pump source for Nd:YAG laser and his relatives were flash lamp.Flash lamp spectrum is broad, while 808 nm laser diode spectrum is much narrower, so the Nd-hosted crystal absorbs most of the power of the laser diode.In addition to that, diode-pumped Nd: hosted laser has many advantages over flash lamp-pumped Nd:hosted laser such as lifetime and compactness.The reason to use 808 nm laser to create 1064 nm laser is the beam quality: 808 nm laser is very powerful, yet its beam quality (especially divergence) ranks as the worst in all laser types.
Since the appearance of this new pump source and the advancing achievements in crystal growth technology, a series of new active medium has been developed: Nd:YVO 4 , Nd:GdVO 4 and Nd:glass, among which Nd:YVO 4 (yttrium orthovanadate doped with neodymium) is the most interesting material.This material has absorption cross section at 808 nm and emission cross section at 1064 nm much greater than that of Nd:YAG [1].This makes the Nd:YVO 4 laser has a much lower lasing threshold than Nd:YAG laser.However, the thermal conductive coefficient of Nd:YVO 4 is smaller than that of Nd:YAG, thus heat management for Nd:YVO 4 is more difficult.

Effect of laser cavity configuration
Laser cavity can be of plane-plane, concave-plane, concave-concave, concave-convex… forms.Each configuration has different stability, efficiency, compactness and other characteristics.One has to base on the application requirements to choose the suitable configuration.
In this study, we use two kinds of configuration: plane-plane and concave-plane.
The former cavity is very compact yet its efficiency is not as good as the latter.Further details are in the result and discussion section.

Effects of laser cavity parameters
Mirror's radius of curvature, cavity length, position of the Nd:YVO 4 crystal within the cavity all affect, more or less, the performance of the laser.The effects do not limit only to the power of the 1064 nm beam, but also many other features.In this paper, we study the effect of cavity length on the threshold pump power (the minimum power of 808 nm beam pumped required for the laser to start emit 1064 nm beam) and slope efficiency (the slope of the input-output line).
The cavity stability [2] is characterized by the G parameter which must satisfy the inequality (2): (1) Where L (mm) is the cavity length (distance between two mirrors), R 1 and R 2 are radii of curvature of mirror 1 and 2, respectively.Cavity with G = 0.5 has good stability (diffraction loss in the cavity is rather small, so with a low pump power the cavity can emit great amount of laser beam); while cavity with G < 0 or G > 1 is unstable (diffraction loss becomes so severe that the cavity can not emit any laser beam at any level of pump power).
Cavities with G =0 or 1 is on the edge of stability, they may emit laser beam, but only at high pump power, and with slight vibration or shock the cavity may cease to emit laser beam completely.
The plane-plane cavity has infinite R 1 and R 2 , naturally it does not satisfy the inequality (2) and can not emit any 1064 nm beam at all.However, in our experiment, it still emits.We will discuss this anomaly in the result and discussion section.For the concave-plane cavity in our study, R 2 is infinite, so the stability condition can be expressed as: (3) Where R 1 is the curvature of the concave mirror.
From (3) we can see that, theoretically, the laser system start can not emit any 1064 nm beam at all when the cavity length exceeds R 1 .
However, in our experiment, the cavity went unstable and ceased emitting 1064 nm when the cavity length is 112 mm.This will also be presented and discussed our paper.
Inside the laser cavity, the 1064 nm beam forms a standing wave.Its fundamental transverse mode varies as described in Figure 1.The distance between the beam waist (the location where the diameter of the laser beam is smallest) to mirror 2 is given by [5]: In concave-plane cavity, R 2 = ∞ so: (5) 0 2  L Which mean the 1064 nm beam waist lies right on the plane mirror.On the other mirror (mirror 1) it has the diameter [2]: Laser beam contour Science & Technology Development, Vol 16, No.K1-2013 Trang 40 From equations ( 5) and ( 7 and when L is approximately to R 1 we will achieve the highest output power.However, the fact is in the opposite.

EXPERIMENT
In our setup, we used a 808 nm laser diode (capable of emitting 20 W beam power) from Spectra Physics,USA to pump the crystal.The laser diode beam was coupled in a bundle of 19 fibers, whose total core diameter is 1100 m, and imaged into the crystal through a lens system which has the imaging ratio 1.6:1.The waist of 808 nm beam inside Nd:YVO 4 crystal is therefore 687.5 m.

A b
In the setup of plane-plane cavity (Figure 3), the active medium was Nd:YVO 4 doped 1%  Why the plane-plane cavity can be capable of emitting 1064 nm, while conditions (1) and (2) state that is impossible?The reason may be the thermal lens in Nd:YVO 4 : part of 808 nm beam absorbed by Nd 3+ ion in YAG lattice does not help generate 1064 nm beam, but wastes as heat.This heat creates a temperature gradient in Nd:YVO 4 and thus a gradient of refractive index.Medium with refractive index gradient bends light that propagates through it, thus acts as a lens.The G parameter of a cavity with internal lens is: From Figure 6, we can see the graph is linear in the pumping range from 0 to around 7 W, this is the good working range of this laser, after that there is a drop in slope efficiency.
Pumping more 808 nm beam caused our crystal to crack and become permanently useless.This is due to the high doping concentration of Nd 3+ .
In laser with Nd:YVO 4 0.27% , this phenomenon did not occur.
In the good working range, the relation between input-output in this plane-plane cavity can be expressed with the equation ( 10): (10)    From the results, we can see that planeplane cavity with Nd:YVO 4 1% can be used to produce compact laser (the cavity length can goes down to 10 mm).However, in terms of output 1064 nm beam power and electrical saving, concave-plane cavity with Nd:YVO 4 0.27% is the better choice: with the same amount of electrical power driven into the 808 nm laser diode, one can acquire more powerful 1064 nm beam from concave-plane Nd:YVO 4 0.27% laser.
In practical usage, we also notice the concave-plane cavity is much more resistant to vibration than the plane-plane cavity.Strong vibration may make the former power's output drop, but only in small amount.In no experience have we ever seen the 1064 nm beam disappeared completely due to strong vibration.Test on misalignment sensitivity needs to be carried out to quantitatively determine the reliability of this laser in harsh working conditions (against shock and/or vibration).This laser, however, is promising in practical usage and commercial production.The 1064 nm beam was tested on several materials and is capable of cutting through plastic and wood.Figure 8 shows etched grooves on plastic and wood.For aluminum and copper in thin layer form, the laser beam can also etch, and the etching threshold (minimum power of 1064 nm beam to etch) is about 0.5 W.However, etching capability on bulk aluminum and copper is very weak.For steel, the threshold power for etching is not high: about 2 W. The low threshold for etching steel may be due to low thermal conductivity of steel compared to silicon: 18 W/m -1 K -1 versus 130 W/m -1 K -1 .Etching steel by 1064 nm beam is much easier than etching silicon, to the degree that there was spark during etching (the steel particles being burnt with atmospheric oxygen), and after 5 minutes of etching a circle groove at the same position on the steel plate, we can cut through and create a hole, as in Figure 9.  Từ khóa: Nd:YAG, Nd:YVO 4 , DPSSL, ngưỡng phát, độ dốc hiệu suất

Investigation in application
Therefore, Nd:YAG laser has been being replaced by Nd:YVO 4 laser in only low and medium output power modules.Science & Technology Development, Vol 16, No.K1-2013    Trang 38The temperature of 808 nm pump laser diode is also very important.The p-n junction, which emits 808 nm beam when injected with electrical current, also emits an amount of heat equal to approximately 50% of the input electrical power.High temperature at the laser diode does not only shorten the life of the laser, but in case of excessively high temperature, can even result in instant death of the laser diode.Active medium temperate also needs decent concern.When absorbing 808 nm beam from the pump laser diode, Nd:YVO 4 use part of it to generate 1064 nm (and then 532 nm) beam, the rest absorbed pump power wastes as heat inside the crystal.The crystal may fracture under steep temperature gradient [2], and the 532 nm output also decreases.Below the fracture limit, temperature gradient still causes bad effect, among which thermal lens [3] is the most annoying.Doping concentration plays very important role in Nd:YVO 4 laser [4] due to the low thermal conductivity of Nd:YVO 4 .In this work, a laser pumped by 808nm laser diode was constructed.The laser operated in continuous wave (CW) mode.The active medium investigated was Nd:YVO 4 crystals.Threshold power and slope efficiency were measured and compared.The output 1064 nm beam was tested on plastic, wood, paper; semiconductors such as amorphous silicon and crystalline silicon wafer; metals such as aluminum, copper and steel.

Figure 1 .
Figure 1.Fundamental transverse mode of 1064 nm beam inside the concave-plane cavity ) and the laser beam contour in Figure 1, we can see the beam has its waist on plane mirror and then it diverges with position nearer to concave mirror.Through (7), we can see that when the cavity length increases from 0 to R 1 , the beam diameter on concave mirror increases from 0 to infinitive.According to D.G. Hall [6], to achieve high efficiency, the smaller the ratio l p   / (where l p   , are the waists of the 808 nm pumping beam and 1064 nm lasing beam, respectively) the better.Because the waist of 808 nm pumping beam in this experiment is kept constant, we expected that longer cavity would have lower ratio l p   / and thus give higher power of 1064 nm beam,

Figure 2 .
Figure 2. Fiber-coupled laser diode (a) and the output bundle tip under microscope (b).Note the 19 fibers arranged into a round tip in (b)

(Figure 3 .
Figure 3. Setup of the laser system with plane-plane cavity.1: Diode laser; 2: coupling lenses; 3: Nd:YVO 4 ; 4: output mirror In the setup of concave-plane cavity (Figure 4), the active medium were Nd:YVO 4 doped 0.27% (3×3×12 mm).The crystal is coated antireflection (AR) thin films 1064/808 on both sides.The concave mirror is coated HR1064 and radius of curvature 100 mm.The plane mirror is coated with transmission T=20% at 1064 nm The concave face of mirror 3and the plane mirror 5 form a concave-plane cavity.

FigureFigure 7 .
Figure 7a andFigure 7b shows the graph of input versus output in concave-plane cavity.The first thing to remark is the stability with respect to cavity length.As stated in (3), cavity with L longer than 100 mm (value of R 1 ) can not emit laser beam.However, from the graph, we can see that at even L=100 mm, the cavity still emitted 1064 nm beam, and from the data collected we see that the 112 mm long cavity the waists of the 808 nm pumping beam and 1064 nm lasing beam, respectively) the better for efficiency.The Nd:YVO 4 crystal is placed very close to the concave mirror, because the 808 nm beam waist right there, and we can from the equation (7) see that cavity with longer length has larger beam diameter on concave mirror, thus a lower l p   / .On the contrary, the G parameter approaches unity when L approaches the value of R 1 , the cavity stability decrease with longer cavity.These two opposite trends lead to a compromise: a L value to balance between the l p   , ratio and the G parameter.That is why the 60 mm long cavity showed the best performance.

Figure 8 .
Figure 8. Etched groove on plastic sheet (a) and on wood sheet (b) under metallurgical microscope

Figure 9 .
Figure 9. Photograph of cut-through hole on 100 m steel plate (b)

Figure 10 .
Figure 10.SEM images of etched groove on 200 m thick silicon wafer Figure 10is the SEM images of 11 etched grooves on 200 m thick silicon wafer under different power of 1064 nm beam.The lens used to concentrate the beam power has the focal length 50 mm.From left to right, the powers of the 1064 nm beam are 5 W, 5 W, 5 W, 3.86 W, 3 W, 2.3 W, 1.45 W, 0.75 W, 0.56 W and 0.37 W. We can see only the first six grooves through SEM image, thus the etching threshold for this material is 2.3 W. The existence of this high threshold originates from the local temperature reached.The local temperature is decided by the heat per unit volume of silicon generated when silicon absorbs the 1064 nm and the heat dissipated to the surrounding area.Silicon has higher thermal conductivity than that of wood, plastic, thus a more powerful beam is required to make the local temperature reaching burning point.In this study, we have not yet perform test with lens of other focal length.The etching threshold when using lens of shorter focal length is expected to be lower and vice versa.

Figure 11 .
Figure 11.Photograph of etched circle on amorphous silicon deposited on glass Amorphous silicon deposited on glass substrate is rather easy to etch: 0.5 W of 1064 nm beam can easy etch a circle on it, as seen in Figure 11.Of course, many material properties contribute to the burning under laser beam: specific heat capacity, reflectance and absorbance at 1064 nm, thermal conductivity, combustion with oxygen [7].A decent

Table 2lists
the threshold powers P th of concave-plane cavity at different cavity length.

Table 2 .
Threshold power of concave-plane cavity